Montero was born into a lower-middle class family in Cuatro Caminos, a district of Madrid. A bout of tuberculosis forced her to remain at home between the ages of five and nine, and she began reading and writing extensively during that time. She then entered the Beatriz Galindo Institute of Madrid, and at 17, she began her university studies in Madrid's School of Philosophy and Letters (Facultad de Filosofía y Letras). The following year she was admitted into the Journalism School, and during her university years, she participated in independent theater groups.

After school, she began working as a journalist, and in 1976 she began working at El País. In 1977 she began publishing interviews in the Sunday edition of the paper, and the following year, she won the "Manuel del Arco" prize for her work, and was the first woman to receive it. She published her first novel in 1979, Crónica del desamor (Chronicle of Enmity). In 1980, she won the National Journalism prize for her articles and literary reports, and that year she was named editor in chief of the weekly version of El País.

In 1981 she published La función Delta (The Delta Function), and the following year, a collection of her interviews published in El País was released, with the title "Cinco años de país" (Five Years of El País). The novel Te trataré como una reina (I Will Treat You Like a Queen) followed in 1983 and was a commercial success. She was awarded the World Prize of interviews in 1987, and published Temblor (Tremor) in 1990.

Her first children's story, "El nido de los sueños" (The Nest of Dreams), was published in 1992, and in the following years she released Bella y oscura (Beautiful and Dark, 1993) and La vida desnuda (The Naked Life, 1994). In 1994 she was awarded the Journalism Prize, and in 1997 she received the Spring Novel Prize for her work La hija del canibal (The Cannibal's Daughter). In 1999, she published Pasiones (Passions), and in 2002, Estampas bostonianas y otros viajes. In 2003, she published what she considers one of her best works, La loca de la casa (The Lunatic of the House). This book won the "Que Leer" Prize to the best book published in Spain in 2003, and the Grinzane Cavour Prize to the best foreign book published in Italy in 2004. In 2005 she published "Historia del Rey Transparente" (Story of the Transparent King), which has also won the "Que Leer" Prize to the best book published in Spain in 2005.