Their father and his brother, George Thomson, were shipowners extensively involved in the China Trade.

He succeeded his late brother Richard Renshaw Thomson, whose sudden death left the position vacant.

Thomson was elected as a Democrat to the United States Senate to fill the vacancy caused by the resignation in early 1853 of his brother-in-law Robert F. Stockton.

Thomson was re-elected in 1857, and altogether occupied the seat from March 4, 1853, until his death in Princeton, New Jersey in 1862.

Thomson was a friend of President James Buchanan, and supported repeal of the Missouri Compromise on the grounds that slavery was permitted under the United States Constitution.

Although he was a friend of Buchanan, he deserted the former president and his party after the capture of Fort Sumter by the secessionists.