His uncle George Robert Ainslie was a soldier and colonial governor.

[3] He transferred to the 4th Regiment of Light Dragoons on promotion to lieutenant on 29 January 1826.

[4] He was promoted to captain on 16 March 1830[5] and transferred to the 1st Regiment of Dragoons on 29 June 1830.

[6] Promoted to major on 14 October 1842,[7] Ainslie transferred to the 14th Regiment of Light Dragoons on 3 February 1943.

[15] In 1834, Ainslie married firstly his cousin Jane Anne, daughter of the politician Francis Gray, 14th Lord Gray (his grandmother's brother); he married secondly, in 1843, Lady Sarah Eliza, daughter of Thomas Lyon-Bowes, 11th Earl of Strathmore and Kinghorne.