Martínez obtained his degrees in Industrial Engineering in 1970 and Economic Sciences later that decade. He developed a significant part of his career at the Repsol industrial complex in La Pobla de Mafumet before moving to the general management in Madrid, where he lived for nearly twenty years. Subsequently, he co-founded an industrial consultancy in Barcelona with former colleagues.

Beyond his professional side, Guillem Martínez was passionate about photography and was a direct witness to crucial moments of the late Franco regime. He was the one who captured with a special camera every moment of the Caputxinada, the clandestine act promoted by the then Democratic Union of Students of the University of Barcelona at the Capuchin convent in Sarrià, on the memorable date of March 11, 1966.

In the academic field, Martínez published several research papers focused on aspects of the Spanish Civil War. He also maintained a close and active link with the Centre de Lectura de Reus, where he regularly collaborated in organizing cultural and debate activities.