He was born at Marylebone, London, the son of the British Member of Parliament Middleton Onslow.
He was commissioned as an ensign in the 1st Regiment of Foot Guards in 1787[1] and was promoted to lieutenant in 1793,[2] with the rank of captain in the Army.
[4] The regiment was disbanded the following year, so he was on half-pay when granted the brevet rank of colonel in 1800.
[8] His daughter, Amelia, married Thomas Chamberlayne, who played cricket for Hampshire; their son Tankerville Chamberlayne also had a brief career as a cricketer, and was Member of Parliament for the Southampton constituency three times.
[9] In 1833, Onslow was living at Great Staughton and was appointed High Sheriff of Cambridgeshire and Huntingdonshire.