The Earls of Darnley had been buried at Westminster Abbey, but after the death of John Bligh, the 3rd Earl the vaults were full.The 3rd Earl, left detailed instructions in his will for the construction of a family Mausoleum.

James Wyatt, the most fashionable architect of the time, was commissioned to design a mausoleum. He exhibited the design at the Royal Academy in 1783. A slightly modified design, by George Dance was completed in 1786, but for obscure reasons, it was never consecrated. Humphry Repton started to redesign the gardens for the 4th Earl in the 1790s and the mausoleum became a landscape feature.

The family moved out of the hall in he 1950s though kept the mausoleum. Without the gamekeeper maintaining security the building became prone to vandalism. On 5th November 1980 a blaze in crypt brought down the chapel floor. It continued to decline, and there were schemes proposed to save it, including for example moving it to Shorne Country Park, or the United States. Fortunately, the original plans exist and in 1946, James Wraight RIBA, photographed and made measured drawings of the building facilitating future restoration. In 2001, Gravesham Council purchased it with funding from the Cobham Ashenbank Trust, with the intention of restoration and eventual transfer to the National Trust.

It was built of Portland stone, square with projecting chamfered corners, and surmounted by a pyramid. The body is like a classical temple, with Roman Doric order, fluted columns in antis on the face, prostyle on the angles. There is a flying staircase to the piano nobile. There are lunette openings above cornice. Tomb chests above the angle architraves. On the piano nobile, there is a circular chapel with red marble Corinthian order pillars and coffered dome of plaster. The basement at lower level held 32 coffin shelves under shallow stone dome. 

The Mausoleum  is sited in a clearing amongst woodland in Cobham Park,East of Cobham.