Welling's father is a retired General Motors executive and his mother a home engineer. He has two older sisters and a younger brother, Mark, who is a struggling actor. His father's job often required relocation, first moving from Putnam Valley to Hockessin, Delaware, then settling in Okemos, Michigan at the beginning of Tom's junior year in high school.

He was active in many team sports including baseball, basketball and excelling in soccer, playing the position of goalkeeper. He and graduated from Okemos High School in 1995. After graduation he worked as a construction worker and continue to live with his parents.

In 1998, Tom was discovered by a catalogue shoot scout who suggested he try modeling. After two years, Welling relocated to Los Angeles. He continued to model there while pursing acting.

His first major role was six episodes on Judging Amy in 2001, playing opposite Amy Brenneman, and as a love interest and karate teacher. He later appeared in Special Unit 2 and Undeclared. He was chosen as Clark Kent in Smallville after a nationwide search.

In December 2003 he played the oldest son of Steve Martin in the big screen adaptation of Cheaper by the Dozen. He has now since appeared in the sequel Cheaper by the Dozen 2, and the horror remake of The Fog.

Tom married Caroline Kenter on July 5, 2002, in Martha's Vineyard, after a three year relationship. He currently resides in Vancouver, where the WB series Smallville is filmed.

  Cheaper by the Dozen 2 (2005) .... Charlie Baker
The Fog (2005) .... Nick Castle
Cheaper by the Dozen (2003) .... Charlie Baker
"Smallville" (2001) TV Series .... Clark Kent
... aka Smallville Beginnings (USA: rerun title)
... aka Smallville: Superman the Early Years (UK: promotional title)