Glau, a native of San Antonio, Texas, was classically trained as a ballerina and also studied tango and flamenco. A professional dancer since age 12, Glau relocated to Los Angeles in 2002. She appeared in a few television commercials, but Glau's first television credit was a guest role as a ballerina on the 2002 episode "Waiting in the Wings" (episode 3x13) of the television series Angel. There she caught the eye of producer Joss Whedon, who later cast her in Firefly.

Glau appears on the episode "Love Conquers Al" (1x06) of the TV series Cold Case. She has a small role in the film Sleepover, in which she plays a high school senior named Shelly. She appears in the CSI episode "What's Eating Gilbert Grissom?". She plays Tess Doerner, a paranoid schizophrenic returnee in the second season premiere of The 4400 (episode "Wake-Up Call"), and the season three episodes "The Ballad of Kevin and Tess", "Terrible Swift Sword" and "Fifty-Fifty".

Glau stars in the science fiction film, Mammoth, which was filmed in Baton Rouge, Louisiana, USA, and Bucharest, Romania.

Glau was voted Best Actress of 2005 by readers of SFX magazine for her portrayal of River Tam in Serenity.

Summer has won a 2006 Saturn Award for Best Supporting Actress in Serenity.

Summer has been cast in a new TV movie on ABC Family called The Initiation of Sarah.

On 22 April 2006, Summer stated at the Supanova 2006 convention held in Brisbane, Australia, that she was aware of the 'secret' of the character Shepherd Book from the series Firefly. This has encouraged further speculation of a possible sequel to Serenity.