Méndez began her career in 1972, acting in a movie named Cabalgando a la Luna (Horse-riding to the Moon). She acted in her first soap opera, Paloma, simultaneusly. Subsequently, she was cast on many other soap oeras, and by the mid 1970s, she was a well known superstar throughout Mexico.

During the 1980s, her star kept on growing, as she reached international superstar status. In 1983, she was crowned as Queen of Vina del Mar at the prestigious Viña del Mar Festival in Chile. In 1986, she starred alongside Andres Garcia and Salvador Pineda in international super-hit Tú o Nadie, where she sang the Corazón de Piedra (Heart of Stone) song, which also became a smashing hit, on the Mexican and Latin American music charts. In Tu o Nadie, her brother played a pshysically challenged person. In 1988, she married producer Pedro Torres, and in 1989, she had her son Pedrito, who is her only child so far. Later on, she and Pedro Torres divorced.

In 1990, Mendez came back with another super-hit soap opera, Amor de nadie, alongside Saul Lisazo and Fernando Allende. In Amor de nadie, she played a rape victim who has five men fighting for her love. The soap opera's title song also became a smash hit for Mendez.

After Amor de nadie, Mendez semi-retired from acting, leading a more private life. But even as she did lead a more private life, she was still under the public's eye, and she has been associated romantically with Luis Miguel and Vicente Fox, among others.

The public and gossip magazines have also speculated of a supposed rivalry between her and Verónica Castro. Lucia debutted as a writer of a soap opera in Miami in 1998.

Mendez is rumored to be planning a comeback into Latin American and Spanish United States television channels.

On Saturday, February 29 of 2004, she married for the second time, this time to a person unknown to the public.

On April of 2004, Méndez relocated for six months to Puerto Rico, for filming of a Hollywood movie.