William was born in Princes Street, Lothbury, the only son of Joseph Denison (1726?–1806), who had gone to London from the west of Yorkshire at an early age and amassed a fortune.

William was a highly successful banker and became a senior partner in the firm of Denison, Heywood, & Kennard (based in Lombard Street, London).

[1] Upon the death of his father in 1806, Denison acquired estates in Yorkshire (Seamer, south of Scarborough) and Surrey (Denbies, near Dorking).

Denison also increased the size of his financial inheritance to such an extent that, following his death in Pall Mall, London, on 2 August 1849, the gross value of his estate was an estimated £2,300,000 (equivalent to around £184 million in 2017).

Having never married, virtually all of his estate passed to his nephew, Lord Albert Conyngham, on condition that he take the surname Denison.