Richmond became a British Army captain at the age of 23 in 1787.
On 27 May 1789, while a colonel in the Duke of York's regiment, he was involved in a duel with Frederick, Duke of York, who had expressed the opinion that "Colonel Lennox had heard words spoken to him at Daughbigny's, to which no gentleman ought to have submitted", effectively an accusation of failing to respond to an insult in the way that a gentleman should.
At Wimbledon Common, Richmond fired, but his ball "grazed his Royal Highness's curl"; the Duke did not fire.
Richmond shortly after exchanged his company for the commission of a lieutenant-colonel in the 35th Regiment of Foot.
On 1 July of the same year, he was involved in another duel, with Theophilus Swift, Esq.,
in consequence of a pamphlet criticising Richmond's character published under Swift's name.
They met in a field near the Uxbridge Road, where Swift was wounded in the body, but recovered.

Later in the year he married Lady Charlotte Gordon.
In 1794 and 1795 he participated in naval engagements against the French in the West Indies and Gibraltar, but was sent home when he came into conflict with his superiors.
He was also MP for Sussex, succeeding his father, from 1790 until he succeeded to the dukedom.
Richmond became a British Army captain at the age of 23 in 1787. On 27 May 1789, while a colonel in the Duke of York's regiment, he was involved in a duel with Frederick, Duke of York, who had expressed the opinion that "Colonel Lennox had heard words spoken to him at Daughbigny's, to which no gentleman ought to have submitted", effectively an accusation of failing to respond to an insult in the way that a gentleman should. At Wimbledon Common, Richmond fired, but his ball "grazed his Royal Highness's curl"; the Duke did not fire. Richmond shortly after exchanged his company for the commission of a lieutenant-colonel in the 35th Regiment of Foot. On 1 July of the same year, he was involved in another duel, with Theophilus Swift, Esq., in consequence of a pamphlet criticising Richmond's character published under Swift's name. They met in a field near the Uxbridge Road, where Swift was wounded in the body, but recovered. 
Later in the year he married Lady Charlotte Gordon. In 1794 and 1795 he participated in naval engagements against the French in the West Indies and Gibraltar, but was sent home when he came into conflict with his superiors. He was also MP for Sussex, succeeding his father, from 1790 until he succeeded to the dukedom.
Charles Lennox, 4th Duke of Richmond