On 20 February 1741, he was promoted to brigadier general, and sent to Germany with the British contingent in the War of the Austrian Succession. He fought at the Battle of Dettingen on 27 June 1743, and was afterwards promoted to major general on 13 July. On 30 April 1745, Onslow succeeded Viscount Cobham as colonel and captain of the 1st Troop of Horse Grenadier Guards, while Edward Wolfe took over the King's Regiment. He and Henry Hawley commanded the second line of cavalry at the Battle of Fontenoy. He was one of the generals present in London in November after the British Army in Flanders was recalled to suppress the Jacobite rising of 1745.