In 1944, he joined Mariachi Vargas de Tecalitlán as a violinist and later as a music arranger. In 1955 Fuentes stopped performing in the group, but he has maintained his position as producer, musical arranger and musical director to the present day. As such, he had a profound influence on Mariachi Vargas de Tecalitlán and mariachi music in general. He is best known for his work on Linda Ronstadt's classic, Grammy award winning, multi-platinum Canciones de Mi Padre album. With domestic sales of two-and-a-half million copies, Canciones de Mi Padre stands as the biggest-selling non-English language record in US history.
Rubén Fuentes wrote dozens of Mexican standards, including "La Culebra", "Las Alazanas", "Como Si Nada", "La Bikina", "Que Bonita Es Mi Tierra", "Flor Sin Retoño", "Ni Princesa Ni Esclava" and "Camino Real de Colima" (many composed with Silvestre Vargas). Fuentes also produced several of Linda Ronstadt's traditional Latin albums. His songs have been performed by Vikki Carr, Pedro Infante, Marco Antonio Muñiz and Javier Solis, among others.
Ruben Fuentes was the musical director of RCA Records in Mexico during the late 1950s to the late 1960s. For that reason, he was the arranger and musical producer of many records of major ranchero singers. Most of them were accompanied by Mariachi Vargas de Tecalitlán. Fuentes was also the musical arranger for most of José Alfredo Jiménez's songs, a prolific Mexican-born composer whose compositions, like Fuentes, elevated traditional Mariachi music to international heights and acclaim.