Background
Onslow was the son of Colonel the Honorary Thomas Cranley Onslow and his wife Susannah Elizabeth Hillier, daughter of Nathaniel Hillier of Stoke Park Surrey.
Onslow was the son of Colonel the Honorary Thomas Cranley Onslow and his wife Susannah Elizabeth Hillier, daughter of Nathaniel Hillier of Stoke Park Surrey.
He was educated at Eton College and joined the Scots Fusilier Guards.
He reached the rank of captain and also served in the 11th Regiment. He was a Deputy Lieutenant and Justice of the Peace for Lincolnshire and the East Riding of Yorkshire. He was returned at the next three general elections, holding the seat when Guildford"s parliamentary representation was reduced to one seat at the 1868 general election, but at the 1874 general election he lost the seat to the Conservative Party candidate Denzil Roberts Onslow.
In 1861 he assumed by Royal Licence the additional surnames of Mainwaring and Ellerker.
Onslow died at the age of 68. Onslow married Rosa Anne Onslow, daughter of General Denzil Onslow of Staughton House, Huntingdonshire.
20th United Kingdom Parliament]
In October 1858, Onslow was elected at a by-election as one of the two Members of Parliament (Member of Parliament) for Guildford in Surrey.