Born Hugh Smithson, he assumed the surname of Percy by Act of Parliament along with his father in 1750 and was styled Lord Warkworth from 1750 until 1766.

In 1750, upon the death of his maternal grandfather Algernon Seymour, 7th Duke of Somerset, his father became Earl of Northumberland and changed his name to Percy.

In 1759, he joined the British Army as a teenager and was a captain of the 85th Regiment of Foot by age 17, an achievement that demonstrated the power of wealth and family standing.

As a Member of Parliament and the son-in-law of Lord Bute, Percy was promoted to full colonel and appointed an aide-de-camp to the King in 1764, having barely reached his majority.

When his brigade relieved Francis Smith's demoralized troops at Lexington, Percy carefully organized his forces so as to provide all-around protection.

Even so, William Heath, who led the colonials, managed to surround the retreating British column with fire during a grueling forced march.

When the British found that the bridge over the Charles River in Cambridge was blocked, Percy turned his column down a side road and led them west to Charlestown.

He wrote: During the whole affair, the rebels attacked us in a very scattered, irregular manner, but with perseverance and resolution, nor did they ever dare to form into a regular body.

[clarification needed][8] More substantively, while in command of a detached British force garrisoning Rhode Island he fell into disagreement with General Howe over the feasibility of advancing into the hostile New England hinterland.

Notorious for a bad temper as well as for being one of the richest men in England, the second Duke of Northumberland died suddenly of "rheumatic gout" in July 1817.