William Dawes worked as a Boston tanner and served in the town's militia before the Revolution. He joined the Ancient and Honorable Artillery Company in 1768 and became its second sergeant in 1770. His connections with the Patriot movement made him one of the messengers Joseph Warren trusted as the confrontation with British troops intensified.

On the night of April 18, 1775, Warren sent Dawes to Lexington by the land route through Boston Neck, Roxbury, Brookline, Cambridge and Menotomy. Paul Revere took a separate route across the Charles River. Their purpose was to warn John Hancock and Samuel Adams and carry intelligence of the British expedition into the countryside. Dawes reached Lexington after midnight, then continued toward Concord with Revere and the physician Samuel Prescott. A mounted British patrol intercepted them. Prescott escaped to carry the alarm onward; Dawes also evaded capture but lost his horse and did not complete the journey to Concord.

Dawes's military work continued beyond the famous ride. Commissioned second major of the Boston militia regiment in September 1776, he later worked in military supply in central Massachusetts. A surviving receipt signed at Worcester in July 1780 documents his quartermaster duties. After the war he remained in trade and resumed an association with the artillery company, serving as its clerk when it revived in 1786. He died at Marlborough in 1799. His warning was part of a network of riders, local officers and militias.