Martorell was born on April 18, 1939 in Santurce, Puerto Rico. He is the son of Antonio Martorell II and Luisa Cardona. He is the first of three children. He first started showing interest in arts as a small child with drawings. He studied diplomacy at Georgetown University in Washington, D.C. and then went to study painting with Julio Martín Caro in Madrid. When he came back to Puerto Rico, he started working at the workshop of Lorenzo Homar at the Instituto de Cultura Puertorriqueña (Institute of Puerto Rican Culture). He currently is the Resident Artist of the University of Puerto Rico at Cayey and directs the Ramón Frade Museum at the same institution.

As a writer, Martorell has written books such as La piel de la memoria (translated as Memory's Tattoo by Andrew Hurley), and El libro dibujado (The Drawn Book). He currently writes a column for Escenario, a section of Puerto Rican newspaper El Vocero.

Martorell was the winner of the Bienal de Arte de San Juan, and has illustrated books of several authors including Alma Rosa Flor, Heraclio Cepeda, Nicholasa Mohr, and Pura Belpré. He also illustrated the ABC de Puerto Rico published by Troutman Press; this book was later burned by the Educational Department of Puerto Rico under Governor Carlos Romero Barceló's administration.

Martorell's house in Cayey, Puerto Rico was subjected to arson in November 2006, and hundreds of thousands of dollars in art were lost.

He currently has a workshop in Ponce and another in New York. He is also the father of the dancer Alejandra Martorell and the stepfather of Giovanni Rodríguez, director of the cult film, Red Canyon, and a grandfather of Gael Rodríguez.

The most extensive publication on Martorell's work is Antonio Díaz-Royo's biography Martorell: la aventura de la creación (The Adventure of Creation).