He was born in Reykjavik, Iceland and moved to the United States at the age of 5. Gunnar lived in Maine till he was 11, then his family moved to Texas, where he went to high school and then the University of Texas. He majored in English and mathematics and then went to graduate school in Scandinavian Studies and English. Gunnar Hansen will forever be remembered as "Leatherface" the original slasher from The Texas Chain Saw Massacre (1974). His first job out of high school was as a computer operator. Hansen was a graduate from the University of Texas - Austin. He did some theater work in college. In the summer of 1973 he heard that Tobe Hooper and others were in town and were doing a movie and decided to give it a try. After interviewing with Tobe Hooper and Kim Henkel , the writer of Texas Chainsaw Massacre, he got the part of Leatherface the retarded killer. After Texas Chainsaw Massacre he tried to make a living as a writer. Gunnar worked as a magazine editor for a while and then wrote for a magazine for several years. He have written several screenplays one with his partner Gary Jones (director of Mosquito (1995)). Hansen has also written books and some film documentaries. Gunnar, who is originally from Iceland, also directed a documentary on Greenland. He also spent some time designing web pages for GTE.

During the filming of the dinner scene towards the end of The Texas Chain Saw Massacre (1974), Gunnar's character Leatherface was supposed to cut the finger of Marilyn Burns ' character with a knife. Gunnar was having trouble working the fake-blood dispenser and was very fatigued by the heat and long hours of doing take after take, so unbeknown to everyone but him and Marilyn, he simply cut her for real, which is the take used in the film.Was asked to reprise the role of Leatherface in The Texas Chainsaw Massacre (2003), but declined, claiming that he was insulted by the idea of the original 1974 film, The Texas Chain Saw Massacre (1974), being remade.Worked as a computer operator outside of high school.After acting in the movie _Demon Lover, The (1977), he decided to quit acting and continue his dream of writing, but after being offered many jobs that he refused (including The Hills Have Eyes (1977) and Eaten Alive (1977)) he decided that he could both be an actor and a writer.Said in a TV interview that he is so unlike his The Texas Chain Saw Massacre (1974) character that when people who meet him learn that he played Leatherface, they often react in total shock.Was a football player in high school; also worked as a bar-bouncer prior to acting.Uncle of Kristin Hansen .Shaved his beard off for the role of Leatherface in The Texas Chain Saw Massacre (1974). He never shaved it off again.Uses a chain saw in at least one movie every decade. The Texas Chain Saw Massacre (1974), Hollywood Chainsaw Hookers (1988), Mosquito (1995) and Chainsaw Sally (2004).