Throughout much of the 20th century, the park was known as Riverside Park. It began as a picnic grove in 1870 named Gallup's Grove. It was briefly changed to Riverside Grove, and then eventually Riverside Park in 1912. Prior to 1900, most of the park's patrons arrived via steamship. The Springfield Street Railway extended its line to the park in 1900 and, although Riverside was at the end of the Springfield Street Railway, it was not owned by the railway and is, therefore, not considered a trolley park, contrary to published reports.

In the early 1900s, a few mechanical rides and a carousel were added. The park was purchased in 1911 by Henry J. Perkins who transformed the park from a picnic grove to an amusement park. He built the park's first roller coaster, The Giant Dip, in 1912, which proved to be so popular that another coaster, The Greyhound, was added in 1915. Under Perkins' ownership, the park continued to prosper and additional amusements were added, including a 300-foot-diameter pool that became known as Lake Takadip. The original Giant Dip coaster was replaced in 1920 by a new, more thrilling coaster that was twice the size of the Giant and was eventually named Lightning. A third coaster, Whirlwind Racer, was added in 1928.

The Wall Street Crash of 1929 took its toll on the park and by 1931 it had gone into foreclosure. For the 1932 season, the park operated only Wednesday through Sunday, and in 1933 it closed. Several attempts to reopen the park failed and it remained closed through 1939, although the grounds were occasionally used for company picnics. A drive-in movie theater operated in the parking lot from 1937 to 1939.

Edward Carroll Sr. purchased the abandoned park in 1939 and after making improvements, reopened Riverside Park on May 29, 1940. Carroll is credited with rescuing Riverside and turning it into the largest theme park in New England. He purchased the plans and cars of the 1939 New York World's Fair Cyclone Roller Coaster and opened a new coaster in 1941. That coaster, now known as Thunderbolt, is operating at the park and is the oldest coaster — in its original location — within the entire Six Flags chain of theme parks. The park continued to add new rides and removed some older ones. The theater and bowling alley were removed in the late 1950s, making Riverside Park a seasonal attraction.

Carroll took a liking to auto racing, a sport that was gaining popularity in the Northeast after World War II, and added it to his slate of attractions at Riverside, building the Riverside Park Speedway in 1948, replacing an open-air bandstand. The 1960s was a popular period for stock car racing at Riverside Park. NASCAR began to hold events at Riverside Park Speedway in 1976. In 1968 the park added its first steel coaster called the Wildcat and in 1977, Riverside Park added its first looping roller coaster, The Loop Coaster, later known as Black Widow. After a new roller rink was built in 1978, the former roller rink building, opened in 1916, was demolished in 1979 to expand the park. In 1998, the rink was closed and is now the human resources building. Edward J. Carrol Sr died in 1979, but the park continued to be successful throughout the 1970s and the 1980s. In 1979 the park invested in an Arrow Development log flume known as Red River Rapids. It became the park's first water ride. It was later renamed Poland Spring Plunge. By the 1980s, the park stopped selling individual ride tickets and began charging a "pay one price" admission.

Six years after the opening of Black Widow, the Wildcat was removed and moved to the late Rocky Point Park in Rhode Island. In 1983, Riverside Park added its third roller coaster, which was also the park's second wooden coaster. The owners originally wanted a coaster exactly like the Coney Island Cyclone, but space was limited, so the coaster would need to take up less space and would have sharper twists and turns. It became known as the Riverside Cyclone. In 1987, Riverside attempted to build a white-water rafting ride called the Lost River Water Ride. Plagued with problems, the attraction never opened and was subsequently abandoned. A majority of the ride was demolished in 1989 to make way for Wild River Falls, a waterslide complex consisting of three sets of slides: Riptide, Blue Lightning and Pipeline. A popular attraction, Wild River Falls remained in operation until the opening of the Island Kingdom Waterpark in 1997.

In 1994, Riverside partnered with Lady Luck Gaming in a proposal to build a hotel and dockside casino complex at the park, one of several competing casino proposals in the state. The plan died after Agawam voters rejected a non-binding referendum in support of casino gambling in November.

During the 1996 season, the track on the Musik Express was damaged and the attraction remained closed for a portion of the year. A Chance Chaos was ordered and was scheduled to open for the 1997 season. In December of 1996, the Carroll family agreed to sell the park to Premier Parks of Oklahoma City, Oklahoma. In the Spring of 1997, Premier Parks renamed the park to Riverside: The Great Escape.

Under Premier Parks, various changes were made to the park. For the 1997 season, the new owners invested more than $20 million on general improvements and several new attractions. A new front entrance to the park was constructed, which included the expansion and renovation of Main Street U.S.A. and the relocation and renovation of the carousel. The Island Kingdom Waterpark was an expansion to the park that featured a children's water play area, various slides, and a wave pool. This expansion was added to the south end of the park next to the park's log flume in the location of the parking lot. To make room for this expansion, the parking lot was relocated to the other side of the roadway in former farmland and significantly expanded.

Other additions in 1997 included Mind Eraser (later renamed to The Riddler Revenge) and Shipwreck Falls. In the process of renovation, some older rides such as the Bayern Kurve and The Monorail were removed. The Southern Center midway was rethemed to a 1950s city named Rockville.

In 1998, the water park was further expanded, with additions such as a lazy river, another children's play structure called Hook's Lagoon, a speed slide tower called Cannonball, a family raft slide called Swiss Family Toboggan, and a multi-slide tower called Big Kahuna. The park also added The Hellevator, an S&S Worldwide Turbo Drop tower measuring 21 stories tall, which was originally painted red.

On April 1, 1998, Premier Parks acquired the Six Flags chain of parks from Time Warner. The park continued to be known as "Riverside" until the end of the 1999 season. In 1999, the waterpark was doubled in size. Additions included a new slide tower named Shark Attack and a second wave pool called Hurricane Bay. The park also added a Hopkins river raft ride named Blizzard River to the North End, replacing a set of dry slides and the old Bumper Cars. Blizzard River was themed to incorporate the Penguin character from the Batman franchise; however, the name of the ride was never changed when the park was allowed to use characters from DC Comics on their rides. A new western area called Crackaxle Canyon was added and included four rides, three of which were new to the park. The Riverside Park Speedway was also removed.

In 2000, Riverside was rebranded "Six Flags New England," reflecting similar changes made at other Six Flags properties. On the former site of the racetrack, a new DC Comics-themed section of the park was installed, called "DC Super Hero Adventures." The area featured several new rides, including Superman – Ride of Steel, a hypercoaster designed by Intamin. The park also added Flashback, a relocated Vekoma Boomerang from Kentucky Kingdom, which replaced the Black Widow and Rotor in the North End. Two S&S Worldwide Combo drop tower rides were added next to the Hellevator; the entire drop tower complex was painted white and named Scream. A pedestrian bridge was also constructed over the roadway in front of the park to provide a safer connection between the parking lot and front entrance.

In 2003, the water park was renamed to Hurricane Harbor and saw the addition of the Tornado water slide.

Mr. Six's Pandemonium, a Gerstlauer spinning coaster, was added for the 2005 season in the North End of the park. The Mr. Six name was dropped from the attraction ahead of the 2007 season. At the end of the 2005 season, the Poland Spring Plunge log flume and ferris wheel were removed.

In 2006, Splash Water Falls and Catapult, an S&S Power Sky Swatter, were added to the park, two relocated rides from the recently closed Six Flags AstroWorld. Splash Water Falls replaced the log flume and ferris wheel in the South End of the park, and Catapult was located in the North End of the park near Cyclone.

In 2007, two new children's areas were introduced to the park: Wiggles World and Thomas Town. Wiggles World was located on the site of the former Tiny Timber Town children's section, which saw the introduction of a few new rides and attractions but also consisted of a few rethemed attractions. Thomas Town was located near the front gate of the park and included three rides and a splash pad.

In 2008, Six Flags New England was set to open The Dark Knight Coaster, an indoor MACK Wild Mouse coaster based on the upcoming film of the same name. The park's other Batman-based attraction had its name briefly changed from Batman – The Dark Knight to Batman: The Ride to avoid confusion. However, due to apparent permit issues, the ride was canceled and dismantled, being sent to Six Flags Mexico. The ride would have cost the park $7.5 million in exchange for bringing $280,000 in taxes for the state of Massachusetts. The cancellation angered the city of Agawam, which stated that issues with the ride were not evident. The park then announced the new "Glow in the Park Parade". On November 8, 2008, the Town of Agawam allowed Six Flags the right to build roller coasters up to 200 feet (61 m) in height.

At the end of the 2008 season, it was announced that Superman – Ride of Steel would be rethemed to Bizarro. The track was repainted purple with dark blue supports, and multiple special effects were added; including building structures that give riders the impression of speeding through a city, rings in the shape of Bizarro's S shield that the train passes through, and flame effects. Two new trains with on-board audio were purchased. Bizarro opened on May 22, 2009 at the start of Memorial Day weekend.

In 2010, a new children's area called Mr. Six's Splash Island was added to Hurricane Harbor. This increased the number of children's areas in the park to four. At the end of the 2010 season, Six Flags began removing various intellectual properties from their parks. Wiggles World was rethemed to Kidzopolis ahead of the 2011 season. Thomas Town, because of low guest foot traffic due to its location close to the front gate of the park, was closed following the end of the 2010 season.

In 2011, Gotham City Gauntlet: Escape From Arkham Asylum opened up in the DC Super Hero Adventures section of the park. This ride replaced the football field and was placed on the same plot of land that the canceled Dark Knight Coaster was supposed to be built on. The same year, the "Mr. Six's" portion of Mr. Six's Splash Island was dropped, effectively renaming the area to Splash Island.

In late August 2011, it was announced that Goliath would be added to the park for the 2012 season in the location of Shipwreck Falls, a water ride located in the North End of the park. Shipwreck Falls closed permanently toward the end of the 2011 season. Goliath opened in the Crackaxle Canyon section of the park on May 25, 2012, as well as a new restaurant known as JB's Smokehouse.

A month before opening day in 2013, Catapult was unexpectedly removed and scrapped. The park announced ahead of the season that the former Thomas Town would reopen for the 2013 season as Whistlestop Park. In May of that same year, a new water ride opened in Hurricane Harbor known as Bonzai Pipelines. At the conclusion of the 2013 season, Taz's DareDevil Dive and Twister, a HUSS Topspin located in Rockville, were removed.

New England SkyScreamer opened in 2014 and, at the time of its opening, was the world's largest swing ride at 408 feet in height. The SkyWay attraction permanently closed at the conclusion of the 2014 season.

On July 20, 2014, Cyclone permanently closed. Construction signs were quickly put up, and, during August of the same year, it was announced that Rocky Mountain Construction would transform the ride into a steel hybrid coaster, which would include three inversions and a steeper drop. Wicked Cyclone opened in May 2015.

In 2016, two new attractions were opened: Fireball and Superman The Ride. Fireball, a Larson Loop attraction, was installed in the former location of Twister. Bizarro was transformed back into Superman, which included a new red track color and other effects.

In 2017, The Joker, a 4D FreeFly coaster, was installed on the former site of Splash Water Falls, which closed toward the end of the 2016 season. Mind Eraser, the park's SLC, received new trains with an updated restraint style. Superman The Ride had a limited run of offering Oculus Rift goggles to enhance the ride experience. Buzzsaw closed at the end of the 2017 season.

Harley Quinn Spinsanity opened in May 2018. To make room for it, the Tea Cups were moved to the former location of Buzzsaw and is now classified as an attraction in Rockville. The South End was transformed into Gotham City. This included retheming Mind Eraser to The Riddler Revenge, which received a new color scheme with green track and yellow supports.

For the 2019 season, Cyborg: Hyper Drive, a spinning flat ride, was installed inside the former Hall of Justice building in the DC Super Hero Adventures area of the park. This ride is the first to include the fictional character Grid, a sentient cybernetic system and the arch-nemesis of Cyborg, in Six Flags' DC character roster. Kryptonite Kollider, formerly known as The Joker's Wildcard, was removed at the end of the season.

For the 2020 season, the park announced the addition of Supergirl Sky Flyer, a Zamperla Endeavour located on the site of the former Kryptonite Kollider. As a result of the park not opening for the 2020 season due to the COVID-19 pandemic, the opening of Supergirl Sky Flyer was to delayed to May 2021. The DC Super Hero Adventures section of the park was rethemed to DC Universe, which features newly installed superhero statues and freshly painted buildings.

Goliath was left standing but not operating (SBNO) for the 2021 season and was removed from the park map. In July 2021, a tree fell onto Fireball during a thunderstorm, extensively damaging the attraction and leaving it closed for the rest of the season. Both Goliath and Fireball were scrapped prior to the 2022 season. The plot of land occupied by Goliath was left as a large grass area and known as Crackaxle Commons for the 2023 and 2024 seasons.

Before the start of the 2024 season, Kontiki was removed from the park. Bonzai Pipelines was left SBNO for the 2023 and 2024 seasons, and Typhoon was left SBNO for the 2024 season. Both attractions — as well as Tomahawk, the park's HUSS Frisbee — were removed ahead of the 2025 season.

On August 15, 2024, a replacement for Goliath was announced. Quantum Accelerator is an Intamin Straddle Coaster that will open in 2025, featuring two launches up to 45 MPH and 11 airtime moments.