Thomas John Patrick Welling was born in Putnam Valley, New York. He spent his early boyhood in Wisconsin before moving to Hockessin, Delaware in third grade. His family moved again, this time to Okemos, Michigan, just after Welling's freshman year in high school. There, he lived with his father (a retired executive for General Motors), mother (a "home engineer"), two elder sisters (a schoolteacher and businesswoman, respectively), and younger brother.
During elementary school in Delaware, Welling showed glimpses of his acting talent by appearing in several school plays and pageants until the sixth grade. At Okemos High School in Michigan, he was considered a jock; he developed a passion for basketball, dabbled in baseball, and excelled at soccer (he was third-string goalkeeper for the varsity school team during senior year). He was never particularly studious, however, despite an enjoyment of subjects such as history, and graduated on 4 June, 1995 with "fair" marks. Afterwards, due to his grades, he decided that a university education was not for him. Instead, he decided to enter a more physically challenging occupation, and worked as a construction worker at a warehouse. He continued to live at home and hung out with friends in his spare time.
While on vacation with friends in Nantucket in the spring of 1997, Welling was approched at a bar by an Abercrombie & Fitch shoot scout working for world-famous photographer Bruce Weber. He returned home to Michigan to consider the offer, then headed to Lake George, New York for the shoot. It was the start of a modeling career, and Welling made a few thousand dollars along the way. His photos were published in the 1997 Abercrombie & Fitch Christmas Quarterly.
Welling soon moved out of his parents' home and to Manhattan, where he modeled sporadically for the next two years. He also modeled for the Louisa Modeling Agency, taking jobs from their offices in Munich and Hamburg and modeling for them in Italy, Germany, and Spain. While working in Miami in 1998, he met his future wife, Jamie A. White, a fellow model and two years his senior. They had previously met each other a couple of times during each of their careers.
While he was very successful as a model, Welling ultimately decided that modelling wasn't his career. With encouragement from family and friends, and with the help of contacts he made during his time as a model, he headed west to Los Angeles in January 2000 to start an acting career. He had no formal training and didn't intend to take lessons (he thought it looked easy), but still gave himself a year to decide if it would be a more fulfulling career.
Numerous auditions soon came up, and Welling realized that he actually found the auditioning process quite enjoyable. He started out in television commercials for 7-Up, TJ Maxx, and Verizon, then received a large role in the music video for "Picture Perfect", by Angela Via, in March 2000. With the help of the CAA talent agency, he landed his breakthrough role in late 2000: that of Rob Meltzer, a karate instructor and the title character's 10-years-junior love interest, on the CBS television series Judging Amy. Welling was originally only supposed to be recurring on the show for three episodes, but with the success of the storyline and the good impression he made on the producers during the shooting of the first three episodes, it was increased to a six-episode arc.
In May 2001, Welling guest-starred on the series Special Unit 2, playing Male Victim #1/Flower "Scar" Roberts in the episode The Depths. He also made a cameo in the pilot episode of the series Undeclared, playing a frat boy, in September 2001. The next month, he made his debut in what would come to be his signature role: that of the teenaged Clark Kent/Superman in the series Smallville. He had originally turned down the role twice before accepting it, thinking it would be "lame". However, after auditioning hundreds of other potential candidates, the producers were so sure that Welling had just the fresh-faced look they were seeking that they decided to call him back one last time. He relented and took the audition; he read the infamous "graveyard scene" and loved it. On the way home, he decided to stop at a gas station and call his agent on his cellphone. When Welling's agent told him that he had just received a phone call saying that Welling had won the role, he was overjoyed, and just stood at the side of the road screaming. He stays in hotels in Vancouver (where Smallville is shot) during filming, and spends his free time in Los Angeles with Jamie and his dogs, which include a pug named Cook. He also enjoys visiting with family and friends, running, skiing, and playing basketball during breaks from shooting.
Welling married long-time sweetheart Jamie White on 5 July, 2002 in a private ceremony at Martha's Vineyard attended only by family and close friends. Among the invitees were Welling's co-stars on Smallville, Kristin Kreuk and Michael Rosenbaum. At the groom's request, Rosenbaum sang a special rendition of "Brown-Eyed Girl". Welling wore a custom-made Yves Saint-Laurent tuxedo for the occasion. Following their wedding, the couple relocated to Vancouver, where Welling was slated to start shooting the second season of Smallville in late July. After shooting wrapped up in mid-April 2003, he immediately travelled to Santa Rosa, California, to shoot his part as Charlie Baker in the movie Cheaper By The Dozen, starring Steve Martin and Bonnie Hunt. The movie is scheduled to be released on 25 December, 2003.
During breaks from shooting Cheaper By The Dozen, Welling and Steve Martin would often sneak in games of basketball, though he won't say who was a better player. He has also recently taken up golf, in which he has become quite proficient.