Although he was born in Scotland, Dalrymple's early life was mostly spent in the Netherlands. However, when his father died (in the early 1700s), he returned home, and in 1707, he was elected as one of sixteen Scottish representatives in the newly formed Parliament of Great Britain. His military career flourished at the same time. He became an assistant to John Churchill, 1st Duke of Marlborough during the War of the Spanish Succession, and in 1709, he was sent as an envoy to Augustus II of Poland. The year after, he was promoted to general for his military achievements, but he eventually fell from favour of the royal family, along with his friend, the Duke of Marlborough.
However, when King George I ascended to the throne, Dalrymple was sent as an envoy to Paris, France. For five years in the 1710s, his spies effectively thwarted various "intrigues" by the Jacobites. In 1720, he became Vice Admiral of Scotland, but lost the position in 1733, mainly because of his opposition to the 1733 Excise Bill of Prime Minister Robert Walpole. However, in 1742, when Walpole fell from office, Dalrymple was promoted to field marshal and commanded the "pragmatic" army of Flanders and Germany.
He was given the colonelcy of various units, including the Grey Dragoons (now the Royal Scots Greys), the Black Dragoons (now amalagated into the Royal Dragoon Guards), and the Earl of Angus's Regiment (now a disbanded unit, formerly 1st Battalion, The Cameronians).