His father was a Russian Field Marshal who belonged to the Irish branch of the de Lacy family and had followed James II into exile.

Later in 1757 Lacy bore a conspicuous part in the great victory of Breslau, and at Leuthen, where he received his fifth wound, he covered the retreat of the defeated army.

Soon after this began his association with Field-Marshal Daun, the new generalissimo of the empress's forces, and these two commanders, powerfully assisted later by the genius of Laudon, made headway against Frederick the Great for the remainder of the war.

They were cautious to the point of timidity since Leuthen and many other bitter defeats had taught the Austrians to respect Frederick; but, having resolved to wear out the enemy by Fabian methods, they were strong enough to persist to the end.

Maria Theresa having placed her son, Emperor Joseph II, at the head of Austrian military affairs, Lacy was made a field marshal, and given the task of reforming and administering the army (1766).

But his self-imposed work broke down Lacy's health, and in 1773, in spite of the remonstrances of Maria Theresa and of the emperor, he laid down all his offices and went to southern France.

His active career was at an end, although he continued his effective interest in the affairs of the state and the army throughout the reign of Joseph's successor, Leopold II.