He was a younger son of Brigadier-General Sir James Jefferyes of Blarney Castle, County Cork.
On 20 November 1710, he joined the Army as an ensign in Colonel Heyman Rooke's Regiment of Foot.
He was promoted to lieutenant in Major-General Andrew Bisset's Regiment of Foot on 1 September 1721, to captain on 1 November 1734, and to major on 2 April 1742.
[1] In January 1756, he was promoted colonel commandant of a battalion of the 62nd (Royal American) Regiment of Foot.
Jefferyes distinguished himself in the defence of Port Mahon, particularly in repulsing an attack on the place by storm, though he was taken prisoner.