Dawes was born in Boston to William Dawes and Lydia Boone. He became a tanner and was active in Boston's militia. He demonstrated his opposition to the British by boycotting their products and wearing a homespun suit to his wedding.
On 18 April 1775, the Assassin Connor met with Paul Revere, who was already meeting with Dawes and Robert Newman. Dawes was assigned to ride to Lexington and warn John Hancock and Samuel Adams of the impending attack by the British Army. To ensure that the message would be delivered, Revere and Connor rowed to the Charlestown peninsula at the same time. Dawes took the longer land route across Boston Neck. Using guile to pass the roadblock, he arrived in Lexington in time to warn Hancock and Adams, and was soon followed after by Revere and Connor.
Dawes, Revere, and Samuel Prescott then rode for Concord, but were confronted by British troops. Revere was captured, and Prescott managed to escape and warn the militia. Dawes lost his horse and was forced to hide in a barn for several hours, before making his was back to Lexington.
After the Battles of Lexington and Concord had commenced, Dawes and the militia retreated to the North Bridge. There, he vouched for Connor's abilities to James Barrett, the militia leader. While Barrett read a letter from John Parker, another militia leader, Dawes informed Connor that Revere had been captured, but assured him that Revere would find a way out of his predicament.