As a Lieutenant Colonel in the 15th Light Dragoons serving in Germany in 1762, Erskine performed exceptionally on the field at Emsdorf and was able to present George III with 16 colours captured by his regiment. For his prowess, Erskine was raised to the position of knight banneret, and thus became Sir William Erskine. He came to America in 1776, seeing action at Long Island. Erskine was second in command to Tyron during the Connecticut Coast raid in April 1777. He served in the Philadelphia campaign and then, in 1778, was made Quartermaster-General by Henry Clinton. Erskine went on to lead troops at the Battle of Monmouth and after that campaign was given command of the eastern district of Long Island. Erskine's last active duty was commanding five infantry battalions and a cavalry squadron in pursuit of the Convention Army which was thought to be moving to Virginia in November 1778. Erskine sailed for London in the 1779. Major Baurmeister commented on the void left by Erskine's departure writing, "The army will miss in him an experience and very well-liked man."