Jensen Ross Ackles, better known as simply Jensen Ackles, was born on March 1, 1978, in Dallas, Texas, to Donna Joan (Shaffer) and actor Alan Ackles. He has English, German, and Scottish ancestry. Jensen grew up in Richardson, Texas, together with his older brother, Joshua, and a younger sister, Mackenzie. Jensen graduated from Dartmouth Elementary School in 1990, he graduated from Apollo Junior High School in 1993, and LV Berkner High School in 1996.

Jensen is a sports junkie. He loves football, lacrosse, baseball and basketball. He even played on the baseball and lacrosse teams in high school. The 6' 1" actor first started modeling when he was just 2 years old. When he turned 4, he started appearing in TV commercials for Nabisco, RadioShack and Wal-Mart. He caught the acting bug because he was mostly influenced by his father, who was an actor in Dallas. He used to watch his father study scripts, and that taught him a few things about the industry. During his later years in high school, he started taking theater classes, where he claimed he was the only "jock" in that department. When he was just a sophomore, a friend of Jensen had asked him to attend a local acting seminar. Two guys, Craig Wargo, and an agent, 'Michael Einfeld', were interested in Jensen's talent and wanted him to go to Los Angeles with them.

Jensen had to say no to the offer and admitted at one point, he thought they would forget about him but, eventually, when he went to Los Angeles, he still managed to get help from them. Prior to that, Jensen actually planned to study sports medicine at Texas Tech University and become a physical therapist, before he decided to move to Los Angeles to give acting a try. In 1996, he managed to secure guest roles on several TV shows, which included Wishbone (1995), Mr. Rhodes (1996) and Sweet Valley High (1994). Jensen's big break came when he was cast in the NBC soap opera, Days of Our Lives (1965), as Eric Brady in 1997. He won a Soap Opera Digest Award for Best Male Newcomer in 1998, and was nominated three times in 1998, 1999, and 2000 for a Daytime Emmy Award for Outstanding Younger Actor in a Drama Series for his work on Days of Our Lives (1965). After spending about three years on a soap set, he left Days of Our Lives (1965) and went on to appear in the mini-series Blonde (2001), which was about the life of Marilyn Monroe, playing Eddie G. He also auditioned for the role of Clark Kent on Smallville (2001), but lost the part to Tom Welling, instead.

Not giving up hope, he went for a few auditions and managed to secure a guest role on the popular James Cameron TV series, Dark Angel (2000), as serial killer Ben/X5-493, the brother of main character Max/X5-452, who was played by Jessica Alba. His character died in the episode, but Jensen eventually returned to the show as a regular in the second season as Ben's clone, Alec/X5-494 and continued on until the show's cancellation in 2002. In 2003, he joined the cast of Dawson's Creek (1998), playing the role of C.J., Jen Lindley's lover. He also filmed episodes of the TV series, Still Life (2003), playing the role of Max Morgan, not knowing that the series was actually dropped. He also had a small role in the short film, The Plight of Clownana (2004), playing the role of Jensen. That same year, he was offered the part of Eliza Dushku's love interest on the second season of Tru Calling (2003). Jensen, however, turned down the role which was later offered to another actor, Eric Christian Olsen. He was subsequently cast on Smallville (2001), as Assistant football coach Jason Teague, the new love interest of Lana Lang. In 2005, Jensen managed to earn a lead role in the movie, Devour (2005), playing the role of Jake Gray. Jensen also earned the opportunity to work his father, actor Alan Ackles, who happened to play his character's father, Paul Kilton. The movie, however, received mixed reviews from the public.