His military service began with purchase of a cornetcy in 1st Dragoon Guards (1 August 1752). He served in the Seven Years’ War (1756–1763), initially as a Captain in the 15th Light Dragoons, then transferred to 18th Light Dragoons. He subsequently saw action in Germany as aide-de-camp to Lord Granby. He continued to rise in rank during the peace, first to major in the 3rd Foot on 17 May 1763, then to lieutenant colonel of the 19th Foot on 17 June 1767. He transferred to the 17th Foot on 26 October 1775 and served with this regiment during General Howe’s early campaigns of the American War of Independence.
===Battle of Princeton===
Mawhood was left in command of a force at Princeton, New Jersey by Lord Cornwallis in early 1777 while Cornwallis chased after George Washington’s army after the Battle of Trenton. After Cornwallis’s attack in Trenton was stopped, Washington sneaked his army around that of Cornwallis and attacked the Princeton garrison. Mawhood acquitted himself well in the battle, but much of the garrison was lost, and the British were driven out of most of New Jersey. Mawhood continued to serve in North America, seeing further action during the 1777-1778 Philadelphia campaign.
Highly esteemed in England following the battle of Princeton, he was chosen to raise a new regiment, the Royal Manchester Volunteers (72nd Foot), for service at the besieged garrison of Gibraltar. Mawhood died during the siege on 29 August 1780 after suffering from a gall-stone.