Mendes was born in Miami, to Cuban-American parents and was raised in Los Angeles by her mother after her parents' divorce; Mendes has said that her mother "suffered so much to make my life OK" during her early years. Her father was a car salesman and her mother, also named Eva, is now an elementary school principal. She is the youngest of four children (she has two sisters and a brother). She was raised Roman Catholic and aspired to be a nun. As a child, Mendes was teased about her "buck teeth".

As a child she lived in Miami, Florida, and Los Angeles, California. She never saw herself as an actress while she was young and decided to pursue other interests. She attended Hoover High School in Glendale and later attended California State University, Northridge to study marketing, but dropped out after meeting boyfriend Zedrick Threatt, who helped her to begin an acting career. She studied with Ivana Chubbuck.