Clinton proved a gifted officer, quickly moving through the ranks by purchasing higher commissions. The Dukes of Newcastle certainly helped along the way. .
Interesting aspects of Clinton's life were the debts that his father had after he died on July 10th, 1761. Discussions with the Board of Trade over his father's unpaid debts lasted for years. Attempts to sell the land were futile. Even after the Revolution, his children were unable to recover and compensation for the land. (Willcox, pp.20-21) Clinton's mother passed away in August of 1767, unfortunately known to have a history of mental instability. Love found on February 12th, 1767 when Clinton married Harriet Carter. The couple conceived and loved five children, Frederick, Augusta, William, Henry Jr, and Harriet. William and Henry became prominent members of the military as well. Harriet Clinton passed away on August 29th, 1772, eight days after giving birth to their fifth child, Harriet Clinton. Grieving for over a year, Harriet's sisters took over the care and well-being of his children. .
Clinton had climbed the military ranks to be a lieutenant colonel in the 1st Foot Guards, later renamed the Grenadier Guards. The Guards are an infantry regiment and the most senior regiment of the British Army. The 1st Foot Guards deployed to Germany to fight in the Seven Years War, arriving at Bremen in July of 1760, operating under General Henry Conway near Warberg. The next battle for Clinton took place at the crossing of the Diemel, near Waburg. Led by Prince Ferdinand, the Hereditary Crown Prince fought the Battle of Villinghausen and won for the British. Promoted to colonel, in the Battle of Friedberg he had an injury, and returned to England where he continued his military career, becoming colonel of the 12th Regiment of Foot. In 1772, he entered Parliament on behalf of his cousin, the 2nd Duke of Newcastle; in the same year, Clinton accepted a promotion to major general, and in February 1775 he became third in command of British forces in North America under Thomas Gage and William Howe.