reed was elected as a democratic-republican to the united states senate in 1806 to fill the vacancy caused by the resignation of robert wright. he was reelected the same year and served from november 25, 1806 to march 3, 1813. although he voted, on june 17, 1812, against declaring war on britain, reed served as a lieutenant colonel of the twenty-first regiment of the maryland militia and later as lieutenant colonel commandant. he led a successful defense in the battle of caulk's field in august 1814.