The site began in 1918 as the Sunken Gardens - landscaped lawns and bedding plants laid out on land reclaimed from the shingle, in the Edwardian seaside tradition. In the 1920s the proprietor, a Mr Maxwell, added a small selection of children's rides. Maxwell was also one of the early co-directors of EKCO, the radio and electronics manufacturer that became Southend's biggest employer between the wars. The little garden survived in something like that form for fifty years - a place for ice-cream and helter-skelters at the seaside end of an East End day-trip. In 1976 the Miller family bought the land west of Southend Pier and began the long expansion that converted it into a full amusement park. For some years it was known as Peter Pan's Playground, then Peter Pan's Adventure Island, and eventually just Adventure Island. The Millers still own it through their company Stockvale Limited.

In 1995 the family bought the land east of the pier as well, doubling the available footprint. Adventure Island now wraps around the foot of the longest pleasure pier in the world - 1.34 miles out into the Thames Estuary, far enough that the trams at the end seem to be sailing in their own private sea. The park does not divide itself into themed lands. Instead it organises rides by minimum height: no limit, one metre, 1.2 metres, 1.3 metres. Until 2021 it sold colour-coded wristbands - red for under one metre, green for under 1.2, blue for over - so families could quickly tell which rides were available to which child. Today the system is integrated into the entry pricing, and a single wristband admits to whichever band you qualify for.

The east side of the park is dominated by Rage, the Gerstlauer Eurofighter 320+ coaster that opened in 2007 on the former Raging River Log Flume site at a cost of around £3 million. With its 97-degree drop, 361-metre track and three inversions, it is the park's flagship thrill ride. Mighty Mini Mega is a Pinfari steel coaster originally built on the ground in 2003 and later raised onto the roof of the Mega City arcade for added height. Green Scream is a family coaster with no inversions. Kiddi Koasta is for the youngest visitors. Barnstormer is for those who are not quite ready for Rage. Between them sit the more sedate fixtures - the Sea Dragon roundabout, the Smiles-Per-Galleon pirate ship, the Pirate Plunge drop tower, the Jungle Express mini-train, the Fireball spinner, the dodgems (which closed in October 2022), and a Mini Miami ride themed - charmingly, baffingly - to the nursery rhyme "Hey Diddle Diddle."

Adventure Island has had to navigate some bruising public moments. In September 2020 the park closed early following police advice about a Travellers' encampment in the nearby Kursaal car park. Its Facebook post used the phrase "traveller invasion" - language that Traveller rights organisations criticised as reinforcing prejudice against a community that has faced centuries of discrimination across Britain. The post was deleted; the company apologised. In 2021 the park hosted its first Pride event with local drag queens performing. In July 2023, after a controversial performance by the drag queen Crystal involving an angle grinder, owner Philip Miller announced Adventure Island would not host future Pride events, citing the park's family focus. Crystal accused the park of corporate retreat from a community it had invited in. Both sides made their case publicly. The park kept running its other annual events - Fairytale Weekend, Superhero Weekend, the October half-term Halloween week, Christmas Wonderland, and Islandfest, which showcases local musicians and artists.

Southend-on-Sea has been London's nearest seaside resort for two centuries, an hour by train from Liverpool Street to a pier that stretches further out into the estuary than any other pleasure pier on Earth. The town has changed shape repeatedly - genteel Victorian resort, working-class day-tripper destination, the Kursaal funfair in its hey-day, decline through the 1970s, slow renewal in the 2000s. Adventure Island has tracked that whole arc, run for a century by two different family proprietors who have kept rebuilding the park to match what visitors actually wanted. VIP Grinders named it Britain's best-value theme park in 2024 and 2025. From the seafront the rollercoasters frame the silhouette of the pier; from the end of the pier the rides look small and busy against the lights of the town. It is one of the few large independent amusement parks left in Britain - the rest mostly belong to international chains. Adventure Island still belongs to the Millers, and to Southend.