She is best known for her lead role as Juliet Burke on the ABC mystery drama series Lost (2006–2010), for which she received a Primetime Emmy Award nomination.

[3] Mitchell also had lead roles on the television series V (2009–2010), Revolution (2012–2014), Dead of Summer (2016), and The Santa Clauses (2022–present), for which she received a Children's and Family Emmy Award nomination.

[5] She attended Stephens College, graduating with a bachelor of fine arts in acting, and then studied at the British American Drama Academy.

Mitchell was later named the lead actress on V[18] and her character was indeed written out in the premiere of the sixth season of Lost, but returned for the two-part series finale.

[27] In 2016, Mitchell starred as U.S. senator Charlene "Charlie" Roan in the science-fiction horror film The Purge: Election Year.

Also that year, it was announced Mitchell would join the main cast of the Freeform supernatural horror series Dead of Summer, portraying Deb Carpenter.

[12] In 2022, she joined FBI: International in the recurring role of Angela Cassidy, a government agent and the long-missing mother of main character Scott Forrester.

[34] While filming The Linda McCartney Story in 2000, Mitchell and co-star Gary Bakewell began dating and later became engaged, but the relationship ended in 2002.