J. Martínez Lozano was born on 29 March 1923 in the El Born district of Barcelona.
As a child, Josep showed no special leaning towards the plastic arts or painting. But as he grew up he started to paint and his vocation finally took root. Like so many other artists with a somewhat bohemian or eventful lifestyle, he had to fight family opposition against his determination to devote himself body and soul to painting. As he himself recalled: "I told my father: I want to be a painter. He replied: In that case, you can leave this house. And I did". From then on he always put painting first, before everything else in his life.
His earliest lessons were at the Escola d´Arts i Oficis (School of Arts and Crafts) in the Barcelona district of Clot (1937). There he learned his first, basic notions of drawing from Doménech Soler, a teacher from Sabadell near Barcelona. He would later become a disciple of Ramón Sanvicens and Joaquim Terruella.
He did his military service at Figueres, a town near the Spanish border, shortly after the end of the Second World War, when Spain was undergoing its own difficult post-war era.