The area has been recognised as important for wildlife and conservation since the 1950s, when the Nature Conservancy had an interest. It is now an internationally important site and is designated as a Site of Special Scientific Interest, a Special Protection Area and a Ramsar site. This is because of its variety of habitats, from grassland and woodland to salt marsh and mudflat. These habitats provide food and shelter for overwintering wildfowl: dark-bellied brent geese, grey plover, knot in internationally important numbers and dunlin, redshank and ringed plover in nationally important numbers. The park also has 11 species of nationally scarce plants and 30 rare (Red Data Book) species of invertebrate .

The Chairman of Benfleet Urban District Council suggested a country park for the area in the 1930s. The Council bought Benfleet Downs during this time for such a purpose. In 1964, the Essex Development Plan proposed a country park, although the idea had been around since WW2. In 1966, a golf course was proposed for the centre of the area. Conservation bodies opposed, stressing the educational value of the site. The 1968 Countryside Act allowed local authorities to acquire and maintain areas as country parks.

In 1970 draft proposals for the establishment of a country park were sent out to the local authorities involved. A public inquiry in 1973 considered these proposals and also an application by the Salvation Army for a massive housing estate on their land. The result of the inquiry made in 1974 turned down the housing proposal, but gave approval for the establishment of a Country Park. The plan was for it to cover all of the land south of London Road between Leigh and Benfleet stations and as far as the creek, the Belton Hills and the western part of Two Tree Island - a total of about 1585 acres. Poynetts and Kersey farms had been bought by Essex County Council in 1973. The Benfleet Downs were given by Benfleet Urban District Council. Southend Borough Council gave Belton Hills and the western end of Two Tree Island. Eventually, the three authorities bought 21 acres of land from British Rail to complete the park. In 1987 the Country Park was officially opened by Sir Derek Barber, Chairman of the Countryside Commission. Although the Country Park takes its name from the nearby castle, the Ranger Service do not manage the Castle. This is the duty of English Heritage, the government body for monuments.

In 1990, a road link from Canvey to Southend was proposed. It would have crossed the creek and marshes on an elevated carriageway to a roundabout below the Castle, and then on to the A13, just west of Tattersall Gardens. This proposal was vigorously opposed by local residents who mobilised into a group called “Save Hadleigh Castle Country Park”. In 1991, the road proposal was turned down, and the group disbanded. Members from the group went on to form the “Friends of Hadleigh Castle Country Park”. This group is still active today.

The Country Park is managed by Essex County Council Ranger Service. The three staff look after the Park’s 376 acres of grassland, marshland, seawall, woodland and scrub. The site is managed for the benefit of its wildlife, as well as its human visitors.