Martínez began her career in Spanish cinema in 1976, with awarded movie And They Lived Happily Ever After. She then went 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.