Smallville
Welling was cast in the WB superhero drama Smallville after a nationwide search for an actor to play Clark Kent. In an interview with a teen magazine, Welling spoke about the day he auditioned for the role: "I was on my way back from the Warner Bros. studio, and I stopped in a gas station to call my manager and tell him how it went. I called and got him on the phone and he said, 'Can you hold on a second?' Next thing I know, there are literally seven people on the other line and almost in unison they say, 'Tom, you got it!'" The pilot aired in October 2001 and became the highest-rated debut for The WB, with 8.4 million viewers. Welling told TV Guide that he turned down the lead role twice, but after reading the script, decided to take the job. Like Christopher Reeve, he was not a Superman fan before being cast as Clark Kent. In fact, he said, he continued to not read Superman comics: "I made a conscious decision to stay away from that material. We're doing something different at a time before all that, I don't want that to affect what I'm doing, even subconsciously." He did get the chance to meet two previous Superman actors: Christopher Reeve, who appeared in season 2: episode 17 titled "Rosetta", and Dean Cain, who appeared in season 7: episode 4 titled "Cure".
Welling was named one of People magazine's "Breakthrough Stars of 2001", and also won the Teen Choice Award as "Choice Breakout Star (Male)" in 2002 for his role as Clark Kent. Following the third season, Welling was interviewed by Smallville Magazine and said that if he could play one character on the show that was not Clark Kent, it would be Lex Luthor; "Lex Luthor! I wouldn't mind having my head shaved—as an actor, [it is the ultimate,] getting to play the complete opposite of your character."
In 2003, Welling said he met with director Brett Ratner about potentially playing Clark Kent/Superman in the superhero film Superman Returns (2006), but scheduling conflicted with Smallville. Comic book artist Alex Ross even did two sketches of Welling as the "Man of Steel" to see what the actor would look like in the famous costume. In August 2009, Welling won another Teen Choice Award as "Choice TV Actor (Action Adventure)" for Smallville.
In 2019, Welling reprised his role as Clark Kent in the Batwoman episode of the CW Arrowverse crossover event "Crisis on Infinite Earths", along with his Smallville co-star Erica Durance as Lois Lane. Welling's reprisal concluded his incarnation's story where it showed Clark gave up his powers, retired as an average farmer, and has at least two daughters with Lois.
On July 13, 2022, Welling and fellow Smallville actor Michael Rosenbaum launched the podcast Talkville, where the two co-stars comment on every episode of Smallville.
