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

In 1546, he granted the manor to Sir Richard Morrison, who started to build Cassiobury House in the extensive gardens, but had not made much progress by 1553, when he went into exile abroad.

After the death of his father in 1556, Sir Charles Morrison (1549–1599) continued building and completed the mansion, which had 56 rooms, a long gallery, stables, a dairy, and a brewhouse.

He moved the Capell family seat from Hadham to Cassiobury, commissioning a lavish reconstruction of his father's Tudor mansion from the architect Hugh May.

As he had already done at Windsor Castle, May teamed up with the wood carver of the day, Grinling Gibbons, and the painter Antonio Verrio to create a sumptuous interior.

Capell was an ambitious courtier, and by fitting out his mansion with richly decorated state rooms in the style of Windsor, he hoped to attract a visit from the King to Cassiobury — he was not successful.

[7] The diarist John Evelyn visited Cassiobury on 16 April 1680 "On the earnest invitation of the Earl of Essex," and dedicated an insightful passage to the merits and disadvantages of the house and grounds, describing some of the interior fittings and decorations.

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

Only one now survives: Cassiobury Lodge,[12] in Gade Avenue, "... the most elaborate in execution — its whole exterior being covered or cased with sticks of various sizes split in two", wrote a Victorian visitor.

Suffering from chronic illness, Adelaide often moved her place of residence in a vain search for health, staying at the country houses of various British aristocracy.

[13][14] 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 death of the 7th Earl of Essex set in motion events that were to bring about the demise of Cassiobury House and change the town of Watford.

Death duties, a form of taxation introduced in 1894 by the Liberal Government, was placing an increasing financial burden on landed gentry at this time and was responsible for the breaking up of many large estates across Britain.

[17] 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.

The lavish interior fixtures and fittings, furniture and fine art collection were split up and sold to private collectors and museums around the world, mainly in the United States (see below).

Developers such as William King & Co and Charles Brightman acquired valuable land around Cassiobury to lay out new streets and build new houses in the vicinity of the new station.

[32] A fireplace and panelling were acquired by Frederick Charles and Muriel Blomfield (a cousin of Lord Essex) for their drawing room at nearby Waterdale House.

A large selection of Gibbons panels and overmantels, including the entire panelling from Lord Essex's 1680 state bedroom at Cassiobury, were purchased by the American newspaper publisher William Randolph Hearst to install at his Hearst Castle property; he used them at his Ocean House mansion, now the Annenberg Community Beach House, in Santa Monica, California.

[36] The Cassiobury House in Bedford, New York, which was constructed from bricks salvaged from the demolished Watford mansion, may also have been fitted with interior panelling from Lord Essex's rooms.

The American property featured in a short television report on NBC New York in 2011, which shows extensive wood panelling, some carved chimney pieces and 18th-century Chinese silk wallpaper.

[38] A set of 12 late medieval stained-glass roundels depicting the Labours of the Months which had been installed in Cassiobury House during the 19th century was purchased by the Victoria and Albert Museum.

[51][52] The Parish Church of St Mary, Watford, 1.24 miles (2.00 km) south-east of Cassiobury House, was the burial place for the Morison and Capel families.

Described by Pevsner as "the chief glory of Watford Church", the chapel is noted for its large, ornate wall monuments by the sculptor Nicholas Stone.

[55] In Saint Mary's churchyard can be found the grave of George Edward Doney, a servant at Cassiobury House, who originated in the Gambia.