[1] John was the son of Peter Sherlock, who had been made a baronet and knight by titular king, James III, the Old Pretender, in 1716.
However, during the Irish Rebellion of 1641, the Sherlock family lost its extensive land holdings in County Waterford.
[2] John Sherlock fought in the Anglo-Spanish War of 1727–1729, a failed Spanish attempt to retake Gibraltar.
From 9 December 1774 to 19 March 1775, John Sherlock defended Melilla against troops of the sultan of Morocco, Mohammed ben Abdallah.
When a Spanish fleet arrived with reinforcements, Sherlock broke the siege and routed the Moroccans.