Guardia was elected Miss Costa Rica in 1978, going on to represent her birth country at the Miss World and Miss Universe contests. The contest had been held in Acapulco, and she received offers by Televisa producer Sergio Bustamante to develop a career there.
Guardia moved to Mexico in 1980; this move proved to be crucial for her show business career. In 1980, she appeared in her first television show, alongside Loco Valdés.
Maribel Guardia has acted alongside some very important actors, including Andrés García, Saul Lisazo, and Joan Sebastian. One of her major soap opera hits was alongside Sebastian. Together, they filmed Tú y yo. One film where she acted alongside Andrés García was Pedro Navaja, where she played the main character's wife. She also did a film that was called El Rey de Los Taxistas alongside Luis de Alba.
Maribel lived for five years with Joan Sebastian and had a son with him.
Recently she has done telenovelas made for children. She has also done a comedy in 2006 that is called Que Madre, Tan Padre as well as the variety show, Muévete.
Apart from her jobs as an actress and singer, she has also modeled for various calendars, which are sold to her fans.