Tom, Thomas or Tommy Johnston may refer to:
- Thomas Jack (Tom Jack Johnston, born 1993), Australian DJ and musician; pioneer of the "tropical house" subgenre
- Tom Johnston (musician) (born 1948), American musician, co-founder of The Doobie Brothers
- Thomas Johnston (engraver) (1708–1767), American engraver and organ builder
- Thomas A. Johnston (1848–1934), American educator
- Thomas Alan Johnston, Scottish engineer
- Thomas Brumby Johnston (1814–1897), Scottish cartographer
- Thomas E. Johnston (born 1967), US federal judge
- Thomas Henry Johnston (1880–?), New Zealand tram conductor, labourer, miner and strike-breaker
- Thomas L. Johnston (1927–2009), Scottish scientist
- Thomas Nicol Johnston (1870–1923), Scottish zoologist
- Thomas Harvey Johnston (1881–1951), Australian biologist and parasitologist
- Thomas Henry Johnston (British Army officer) (died 1891)
- Thomas W. "T. J." Johnston Jr. (born 1956), American lawyer and Anglican bishop