Born into a prominent Boston family, Cushing graduated from Harvard in 1744, practiced law, and worked in his father's accounting firm where he became acquainted with the slightly older Samuel Adams. Elected to the General Court in 1761, he served as speaker during the tumultuous years leading up to the revolution, often being perceived by the British as the face of the colonial dispute. After dissolution of the assembly by Governor Gage in 1774 he was elected to the Massachusetts Provincial Congress, and served as a delegate to both the First and Second Continental Congresses.
The first General Court under the new Massachusetts Constitution convened at the Old State House on October 25, 1780, and Cushing was elected from the new state senators as their presiding officer. Among his duties during his one-week tenure, he delivered the oath of office to his life-long friend John Hancock as the first governor of the Commonwealth, under whom he served as Lt. Governor. He served briefly as Acting Governor during the months between Hancock's resignation and the administration of James Bowdoin, whom he served as Lt. Governor as well.