Sossamon was born in Honolulu, Hawaii, to Sherry Sossamon and Todd Lindberg, and grew up in Reno, Nevada. She is of French, Hawaiian, Dutch, English, Irish, Filipino and German descent; her maternal grandmother is Hawaiian-Filipino, and her maternal grandfather is English-German. The y in her first name was an adolescent addition in 1995. After her high school graduation, Sossamon moved to Los Angeles where she pursued a career in dance. While she attended Dance Unlimited, Sossamon appeared in television commercials and had DJ gigs in local clubs. In 1999, Sossamon was discovered by casting director Francine Maisler, while helping a friend DJ at Gwyneth Paltrow's birthday party.

Her first film role was the female lead in A Knight's Tale. She had five auditions and ended up beating Kate Hudson for the role of Jocelyn. Then in 2002, she played the love interest of Josh Hartnett in 40 Days and 40 Nights. Reportedly close friends with Heath Ledger, she also appeared alongside him in 2003 in The Order, and appeared in music videos for Mick Jagger and Korn. Sossamon continues to DJ weekly at a Los Angeles club, but spends most of her time filming.