It was created in 1909 from the purchase by Watford Borough Council of part of the estate of the Earls of Essex around Cassiobury House which was subsequently demolished in 1927.

[1] It comprises over 190 acres (77 ha) and extends from the A412 Rickmansworth Road in the east to the Grand Union Canal in the west, and lies to the south of the Watford suburb of Cassiobury, which was also created from the estate.

[5] St Albans Abbey claimed rights to the manor of Cashio (then called "Albanestou"), which included Watford, dating from a grant by King Offa in 793.

In 1546 he granted the manor to Sir Richard Morrison, who started building Cassiobury House with the extensive grounds which were much larger than they are today, reaching as far as North Watford and southwards almost to Moor Park.

The Tudor mansion was completed by his son, Sir Charles Morrison, and boasted 56 rooms, a long gallery, stables, a dairy and a brewhouse.

Arthur Capell, 1st Earl of Essex, commissioned Hugh May to rebuild the Tudor house, c.1677–80,[7] with sumptuous interiors created in collaboration with the wood carver Grinling Gibbons and the painter Antonio Verrio.

[8] The park and gardens were laid out by Moses Cooke who devised woodland walks and avenues, and provided "an excellent collection of the choicest fruits".

C. 1799–1805, the 5th Earl of Essex commissioned James Wyatt to remodel the house in the Gothic style, and Humphry Repton to overhaul the park.

In 1841 a fire destroyed the orangery, which was filled with newly collected plants and fine orange trees, some of which had been presented to the 6th Earl by Louis XVIII.

The parties and entertainments at Cassiobury House continued into the new century: in 1902 it was visited by the young Winston Churchill and King Edward VII.

The land was made subject to restrictive covenants stipulating that only good quality detached or semi-detached houses would be allowed.

On Thursday 8 June 1922, at 2.30 p.m. at 20 Hanover Square, "By direction of the Right Honourable Adèle, Countess Dowager of Essex", "Cassiobury Park estate including the historical family mansion, Little Cassiobury, and the West Herts Golf Links, embracing in all an Area of about 870 acres (3.5 km2)" was auctioned by Humbert & Flint, in conjunction with Knight, Frank & Rutley.

Only the stable block remains: this has been converted to Cassiobury Court, a rehab centre for addicts, still extant in Richmond Drive.

Many more recent plantings of exotics have been made, such as swamp cypress Taxodium distichum (L.) Richards and various Asian rowans Sorbus spp.

Man-made structures in the park include a complex of paddling-pools and an adjacent miniature railway near the Gade, tennis-courts, a bowling green, a system of all-weather asphalt paths, and a number of shelters.

The valley is a good place to see willow tit (Parus montanus), reed bunting (Emberiza schoeniclus), and sedge warbler (Acrocephalus schoenobaenus).

In 1997 Whippendell Wood served as a Star Wars filming location when it provided the backdrop of a forest on the planet Naboo which featured in the scene in Star Wars: Episode I – The Phantom Menace, in which Qui-Gon Jinn (Liam Neeson) and his Jedi apprentice Obi-Wan Kenobi (Ewan McGregor) first meet Jar Jar Binks (Ahmed Best).