Caplan was born in Los Angeles, California and grew up in a Reform Jewish family. She attended Alexander Hamilton High School in Los Angeles, where she starred in such productions as Much Ado about Nothing and You Can't Take It with You.

Caplan began her television acting career in 1999, when she played the minor role of Sara on the short-lived series Freaks and Geeks. She had a series of guest appearances on other shows, and has been a guest on Sharon Osbourne's talk show. Lizzy also appeared in Jason Mraz's music video for "You and I Both". Another of her major television characters came on Smallville. She debuted on that show Read more...

Caplan resides in the Hollywood Hills with some roommates. She has a cat named Lisa Turtle, which she named after her favorite character on Saved By The Bell. She is going to be the godmother of friend Busy Philipps' daughter when she is born in August 2008.

Caplan's Cloverfield character, Marlena Diamond, has a Myspace page.