Somerhalder was born in Covington, Louisiana. He has a brother, Bob, and a sister. His father, Robert Somerhalder, was a building contractor of French and English descent, and his mother, Edna, who is of Irish and Choctaw descent, grew up on a pig farm in Mississippi. The surname "Somerhalder" originates from his adoptive great-grandfather. His biological great-grandfather, a wealthy English landowner, got one of his mistresses pregnant and paid an immigrant worker of his to marry this woman and give the baby a name, which was Somerhalder. The biological great-grandfather's name was Hull.

Somerhalder's parents divorced when he was thirteen. He grew up with his mother, whom he has described as being "very spiritual", and attended Catholic school. His family in Louisiana was significantly affected by Hurricane Katrina in 2005 and, as of June 2007, has not been able to recover what it lost from the costliest and one of the deadliest hurricanes in the history of the United States.

Somerhalder began his career as a fashion model, with his mother's encouragement at the age of 10. He modeled in New York, Paris, Milan, and London. His modeling credits include Calvin Klein, American Eagle Outfitters, Dolce & Gabbana, Gucci, Versace, and Guess?. He was the face of Guess? for two fall seasons.

Although he continues to model with his agency DNA Models, he has also started acting.

His first major acting role was in The Rules of Attraction, Roger Avary's adaptation of the Bret Easton Ellis novel of the same name.

His television work includes appearances in shows such as Smallville (in which he played Adam Knight, a dead person who is being kept alive by a special drug), CSI: Miami, Law & Order: Special Victims Unit, CSI: Crime Scene Investigation, Now and Again, and The Big Easy, as well as producing Recess.

His biggest role to date was as a main cast member for the first season of the American television series Lost as the character Boone Carlyle.

Somerhalder returned to play Boone Carlyle in episodes of both the second and third season of Lost. In the second season he guest starred in the flashbacks of Boone's stepsister, Shannon Rutherford in the episode "Abandoned". In the third season, he guest starred in the episodes "Further Instructions" (appearing in John Locke's dreams in the episode) and "Expose" (in flashback scenes that take place during Season 1).

In 2006 he starred in Pulse, a remake of the Japanese horror movie Kairo, with Kristen Bell and Christina Millian. The movie was neither a commercial nor a critical success, performing relatively well at the box office on its first week of release.

In May 2006, DNA Models named their Top 10 Male Models. Somerhalder was among those in the list.

Most recently, Somerhalder guest starred as Nick in HBO's Tell Me You Love Me. His appearance on the show is notable because, in a sex scene with actress Michelle Borth, he appears fully nude.

• Began modeling at age 10, modeled for Calvin Klein, Dolce & Gabanna, Gucci, Versace and Guess (Was the face of Guess? for two consecutive fall seasons).
• Was the face of Guess? for two consecutive Fall seasons.
• Modeled for Calvin Klein, Dolce & Gabanna,and Gucci. However, he doesn't like to talk about his modeling career because of the impressions people might get of him as being just another pretty face.
• Played varsity soccer and baseball.
• Ironically, the first actor to be cast as one of the survivors in "Lost" (2004).
• In 2005, he bulked up, adding about 20 to 30 pounds of muscle to his slim frame, so he would be considered for more leading-man-type roles.
• Has said in an interview he was planning to buy a home in Hawaii when was filming the hit show "Lost" but couldn't after his character was killed off in the show.
• He has a brother, Robert (Bob), and a sister. Their parents divorced when he was thirteen.
• His father, Robert Somerhalder, was a building contractor of French and English descent, and his mother, Edna, who is of Irish and Choctaw descent, grew up on a pig farm in Mississippi.
• Grew up with his mother, whom he has described as being "very spiritual".
• Attended Catholic school.
• The surname "Somerhalder" originates from his adoptive great-grandfather. His biological great-grandfather, a wealthy English landowner, got one of his mistresses pregnant and paid an immigrant worker of his to marry this woman and give the baby a name, which was Somerhalder. The biological great-grandfather's name was Hull.

On his reasons for becoming an actor. "To explore different parts of yourself and different emotional lives....not to hide from who you are but to actually explore who you are". "I have a million acquaintances but just two or three true friends. I can't hide anything from them." "I kind of think too much, I try do too many things at once." On his biggest weakness "This program teaches people that it only takes a single unlucky moment to lead to tragedy, if you come into contact with one really f***ed-up person, they can truly take your life away." In regards to the Matthew Shepard killing and MTV's Fight for Your Rights: Take a Stand Against Discrimination campaign. "You can't get anywhere without the people who have come into your life."