Mendes was born in Miami, Florida (although many sources incorrectly state her birthplace is Houston, Texas) to Cuban parents and was raised in Los Angeles. She went to Hoover High School in Glendale, California. She later attended California State University, Northridge but dropped out to seek an acting career. Studied with Ivana Chubbuck.

Mendes worked her way up through commercials to music videos (including an appearance in Hitch co-star Will Smith's video "Miami") to guest roles in television soap operas and supporting film roles. She has appeared in such films as Training Day, Stuck on You, Once Upon a Time in Mexico, and as the female lead in the comedy Hitch opposite Will Smith. Her role in Once Upon a Time in Mexico also gave her a nomination at the Teen Choice Awards. She has two major movie projects pending, including Ghost Rider, in which she costars along with Nicholas Cage.
Mendes is a spokesmodel for Revlon. She has also worked in interior design and has written children's books. Maxim ranked her #27 in their 2006 Hot 100 issue.

Mendes has been in a long-term relationship with Peruvian filmmaker George Gargurevich.