In December 2003, Welling made his feature film debut as Charlie Baker, the oldest son and second oldest in the Baker family which had 12 children, in the family comedy Cheaper by the Dozen with Steve Martin and Bonnie Hunt, which is a remake of the 1950 movie starring Clifton Webb and Myrna Loy, based on the 1948 book about the family of Frank and Lillian Gilbreth and written by two of their children.
Welling talked about why he was interested in the project in an interview with Paul Fischer: "The top three reasons I decided to do this film were, one, Steve Martin, two, Steve Martin and three, Steve Martin. Then, after reading the script, I really liked this character, Charlie.
'" Welling told TV Guide that he turned down the lead role twice, but after reading the script, decided to take the job.
In 2000, Welling appeared in Angela Via's music video "Picture Perfect" where he played Vía's love interest.Welling has stated that he did not like modelling and that he was not good at it because it was all on the outside or from an external point of view.He pursued acting because he wanted to express a range of emotions, In his first major acting role, Welling played karate teacher Rob "Karate Rob" Meltzer, a younger love interest of Amy Gray (Amy Brenneman) in the second season of the CBS legal drama Judging Amy, which aired in 2001.I liked what he went through, what he had to go through, and I felt I understood where he was coming from.And I just wanted to be a part of it." At the same time The Fog was in production, Welling was still working on the last few episodes of the fourth season of Smallville.