Charles Lennox was educated at Westminster School and succeeded his father to the peerage in 1750. He had many sisters, including the Ladies Caroline Lennox, Emily Lennox, Louisa Lennox and Sarah Lennox. He was admitted a Fellow of the Royal Society on 11 December 1755.
From 1756 to 1758 Charles Lennox was the Colonel of the 33rd Regiment of Foot. In 1757 a second battalion (2nd/33rd) had been raised and in 1758 this battalion became an independent regiment, the 72nd Regiment of Foot. Charles Lennox was appointed Colonel of the new regiment and his younger brother George Lennox took command of the 33rd Regiment (1st/33rd). The 72nd Regiment was disbanded in 1763 at the end of the Seven Years' War.
Charles Lennox was appointed British ambassador extraordinary in Paris in 1765, and in the following year he became a secretary of state in the Rockingham Whig administration, resigning office on the accession to power of the Earl of Chatham.
In the debates on the policy that led to the War of American Independence Richmond was a firm supporter of the colonists; and he initiated the debate in 1778 calling for the removal of the troops from America, during which Chatham was seized by his fatal illness. He also advocated a policy of concession in Ireland, with reference to which he originated the phrase "a union of hearts" which long afterwards became famous when his use of it had been forgotten. In 1779 the duke brought forward a motion for retrenchment of the civil list; and in 1780 he embodied in a bill his proposals for parliamentary reform, which included manhood suffrage, annual parliaments and equal electoral areas.
Richmond sat in Rockingham's second cabinet as Master-General of the Ordnance; and in 1784 he joined the ministry of William Pitt. He now developed strongly Tory opinions, and his alleged desertion of the cause of reform led to a violent attack on him by Lauderdale in 1792, which nearly led to a duel between the two noblemen. Richmond died in December 1806, and, leaving no legitimate children, he was succeeded in the peerage by his nephew Charles, son of his brother, General Lord George Henry Lennox. The adjoining towns of Richmond and Lenox in Massachusetts were named in his honor.
He became a Privy Counsellor in 1765.