He wrote numerous books, stories, and plays, including a theatrical adaptation of Oscar Lewis's The Children of Sanchez.
Graduating from the National Autonomous University of Mexico (UNAM) in 1959 with a degree in civil engineering, Leñero soon turned to writing to support himself.
His first novel, La voz adolorida (1961), exhibits the psychological realism of his early writings, consisting of a mentally ill patient's monologue about his life before entering an asylum.
He was influential in starting the documentary genre of theater in Mexico, and two of his notable works are Pueblo rechazado and El juicio.
Leñero worked as a screenwriter for El callejón de los milagros, film for which he received an Ariel Award in 1995 by the Mexican Academy of Film and El crimen del Padre Amaro (2002, The Crime of Padre Amaro), one of Mexico's highest-grossing movies.