Born in February 1731 at Grangemellon, Kildare, Ireland, he was the fourth son of Sir John St Leger, who was a judge of the Court of Exchequer, and his second wife Lavinia, daughter of Kingsmill Pennefather of Cashel.
[3] In 1761, St Leger married a Yorkshire woman, Margaret Wombwell.
That same year he was appointed lieutenant-colonel of the 124th Regiment of Foot, but a year later the regiment disbanded, and St Leger took on the Park Hill estate in Firbeck, where he later bred and raced horses.
In 1779, St Leger re-entered the army as colonel of the 86th Regiment of Foot.
He subsequently achieved the rank of brigadier general, before serving a period as the Governor of Saint Lucia (1781-1783).