The Manor of Cassio is mentioned in the Domesday Book in 1086, listed as owned by the Abbey of St Albans. It covered an area much bigger than the current park including the golf course, Whippendell Woods and the Cassiobury housing estate.
In 1533, Henry VIII confiscated the Abbey's lands (as part of the 'dissolution of the monasteries') and sold it to Richard Morrison in 1546. It was then that the grand house started to be built although Richard died before it was finished and his son Charles oversaw the completion. The house was passed down the male line until 1628, when Elizabeth Morrison married Arthur, Lord Capel of Hadham.
In 1661, Elizabeth and Arthur's son, Arthur, was made Viscount Malden and Earl of Essex. He employed the gardener Moses Cooke to set out formal gardens, "strictly in the French style" during periods between 1669 and 1680. The earl also commissioned extensive remodelling of the house in the early 1700s.
Despite the first earl's execution for actively supporting the royalist cause in 1649 (toward the end of the civil war) and the second earls arrest for his part in a plot to assassinate Charles II (he died in the tower of London under 'mysterious circumstances in 1683) the estate remained in the ownership of the Capel family and the seat of the Earls of Essex until it was sold in 1922.
When the 6th earl died in 1892, it was clear that despite (or because of) his enthusiasm for new farming projects around the estate no significant maintenance had been carried out on the house in the previous fifty years, consequently many of the family paintings and other valuables were sold to provide funds for restoration.
By 1900, the house had ceased to be used as a permanent residence, and in 1908 parts of the estate were sold off. The Council bought some land at this point and then added more land over the next decades to create the current park. The house was demolished in 1927 and sold off in pieces as building materials. The final purchase was the Whippendell Wood area in 1935. The paddling pools were also built in the 1930s.
In the second half of the 20th century, funding for parks in Watford decreased as part of a general trend across the UK. The resulting decline in management meant that a number of historic features were lost, and the character of the landscape began to change. In 1796 the 4th Earl of Essex was praised for his ‘public spiritedness’ for allowing the construction of the Grand Union Canal through the park. As well as the temporary disruption caused by the building the canal would have had a major impact the park as it was a major trade route in those days (like the M25) not just a quiet leisure route like it is today.
The historic importance of Cassiobury Park was recognised in January 1999 when it was entered at Grade II in English Heritage’s Register of Parks and Gardens of Special Historic Interest. It is also recognised for its ecological value, with the area between the River Gade and the Grand Junction Canal designated a Local nature Reserve in 2003 (Whippendell Woods was designated as a Site of Special Scientific Interest in 1954). Cassiobury Park (including the Cassiobury Park Local Nature Reserve) has been awarded a Green Flag Award for sustainably managed open spaces each year since 2007.
In July 2014, the park received a grant from the Heritage Lottery Fund (HLF) and Big Lottery of £4.5 million (alongside WBC match funding of £2 million). The grant has been used to build a new visitors centre, renovate the pools, modernise the facilities of the Cha Cha Cha, create a new entrance space from Rickmansworth Road and reintroduce and restore the historic bandstand. The money also pays for two new staff to run lots of events and activities. The works completion is estimated at the end of summer 2016 but funding for the staff continues.
The project, which began in 2011, received £6.6m of funding (£5m from the Heritage Lottery Fund and Big Lottery Fund, and £1.6m from the council) and the state-of-the-art and energy efficient Cassiobury Park Hub is now home to additional changing rooms, toilets, fully inclusive Changing Places facilities, events and education space, and the popular Daisy’s in the Park café. The paddling pools also received a £2.8m upgrade as a part of the regeneration works.
Other parts of the project included returning the historic bandstand to the park, improved management of Whippendell Wood and the Local Nature Reserve & restoration of the 18th century Lime Avenue and providing an on site park manager.