Having joined the army's Foot Guards in 1741, he was Colonel of the 2nd Troop of Horse Grenadier Guards from 1745 until 1779. On 11 May 1745 he distinguished himself at the battle of Fontenoy. In 1755 he became a Major-General, a Lieutenant-General in 1758, and finally a 
General in 1770. 
From 1742 he was known as Viscount Petersham until 1765 when he succeeded his father and became 2nd Earl of Harrington. Meanwhile, from 1741 until 1747 he was a Whig M.P. for Aylesbury and from 1747 until 1756 for Bury St. Edmunds. 
In 1746 he married Lady Caroline Fitzroy, daughter of the 2nd Duke of Grafton, and they became the parents of seven children. According to Sir Horace Walpole "she had a very bad character", whereas William Stanhope was nicknamed "Peter Shambler" from his peculiar gait; while 
in "Modern Characters by Shakespear" (1778) he is described as "the genius of famine; and lecherous as a monkey". He seems to have been a person of quite exceptional immorality as in the "Royal Register" for 1778 he is described as "the nobleman who sacrifices all appearance of 
decency and good morals, neglects every domestic duty, every public concern, for the lowest amusements of the lowest brothels." 
He died in 1779 aged fifty-nine while his wife died suddenly of convulsions in 1784 aged sixty-two.

Source: Leo van de Pas