Martinez attended Elk Grove High School, Sacramento City College, Sacramento State University, and Southwest Missouri State University. He was originally signed by the Philadelphia Phillies as an amateur free agent before being taken by the Houston Astros in the 1968 rule 5 draft. The Astros later traded him to the Kansas City Royals. Martinez made his major league debut in 1969, playing 72 games with the Royals. He is mentioned in Jim Bouton's 1970 bestseller "Ball Four" as John Martinez, a player Bouton and his Seattle teammates know little about. During a meeting, as Bouton's team is devising strategies to effectively pitch to their opponents, manager Joe Schultz lacks any concrete suggestions about the rookie Martinez, and famously advises that they just "zitz" him.