Thomas George Johnston (August 4, 1849 – July 4, 1905) was a Canadian physician and politician.

He graduated in medicine from McGill College and took up his father's practice in Sarnia.

He was Mayor of Sarnia for two terms (1896 and 1897) and Chairman of the Board of Health and a School Trustee.

He was first elected to the House of Commons of Canada for Lambton West in an 1898 by-election.

He served in the Canadian militia and participated in the Fenian raids between 1866 and 1871.