Martínez began her career in Spanish cinema in 1976, with the acclaimed movie And They Lived Happily Ever After. She then went on to a small role as a bookseller in Manuel Gutiérrez Aragón's Black Litter (1977), a movie which was awarded in the Berlin International Film Festival.
From 1979 to 1980, Martínez worked on the first period of Barrio Sésamo. She played one of the adult human characters. However, she could not renew her contract for the second season of this first period, and thus Petri simply disappeared from the Barrio, apparently with nobody missing her.
Martínez then disappeared from the screen for no less than fifteen years. She suddenly came back with internationally-awarded movie Entre Rojas (1995), directed by Azucena Rodríguez and starring Almodóvar-favorite Penélope Cruz.
Since then, she has continued working with other Spanish stars, such as Eduardo Noriega in the short feature Allanamiento de morada (Mateo Gil, 1998), where she played a missing person, and Nobody Knows Anybody (again Mateo Gil, 1999), where she played one of the characters providing important clues to Noriega. Both of the movies won widespread recognition and awards, although Martínez's participation did not.
However, her career was increasingly active for a mature actress. She acted in the short Frasquito and in Pedro Teleche's El invierno de las anjanas in 2000.
During 2001, Martínez had a chance to act in different movies, including Jorge Iglesias' comedy Fish People (where she makes a cameo at the park, next to Fernando Chinarro), and award-winning dramas Savages (Carlos Molinero) and Mine Alone (Sergi López). She also, for the first time in her career, had a chance to take top billing inn a movie, in Antonio Muñoz de Mesa's short, El Diskette.
As a rest, she only appeared as a guest in TV series for all 2002, but they were popular TV series: Hospital Central and Policías, en el corazón de la calle.
2003 would mean a big-time return after her holiday. She appeared as a recurring guest star on the TV series Código fuego and played the mother of star Óscar Janeada in Achero Mañas' November. She also was top-billed in Xavi Sala's short 60 years old, and co-top-billed with Manuel de Blas in Gaizka Urresti's dramatic short Raíz.
In 2004, she played in three movies, in almost of the genres an actor could hope for. Firstly and most recognized, she was the mother in Pedro Almodóvar's super-production, Bad Education. She also appeared in Daniel Múgicas' thriller Matar al ángel, and played an unhappily-married woman in Javier Balaguer's comedy Escuela de Seducción, starring Victoria Abril, Javier Veiga, and José Manuel Cervino as Martínez's husband.
Now a veteran actress, she usually shares a set with some of the most recognized actors in Spain, such as Rafael Álvarez 'El Brujo' in Angosto (2006), and also had a role in Javier Rebollo's Locos por el sexo (2006). She had recurring or regular roles on several series, including telenovelas and the sitcom La que se avecina (with José Luis Gil), as crazy old neighbor Doña Fina beginning in 2014.