His father was Primo Barreto, a clarinetist who taught music to all of his children: Lita, Josefina, Estela, Alejandro "Coco", Roberto "Bobby", and Guillermo.

[6] This swing background would allow him to take his music into the realm of Afro-Cuban jazz as part of the Quinteto Instrumental de Música Moderna, which he founded in 1958 alongside Frank Emilio Flynn.

Inspired by bop drummers like Max Roach and Roy Haynes, by the early 1950s, Guillermo would organize the Sunday afternoon jam sessions (known as descargas) at the legendary Cabaret Tropicana (featuring stars such as Bebo Valdés[nb 2]), often doing the transcription necessary to explain American jazz music to his bandmates to play.

By the early 1960s, he was amongst the most prolific drummers in the Cuban jazz scene, playing with the likes of Chucho Valdés, Fernando Mulens, Peruchín and, since 1967, the all-star big band known as Orquesta Cubana de Música Moderna.

[6] The Quinteto Instrumental de Música Moderna, "which quickly gained stature as a benchmark in Cuban Latin jazz history",[9] would later evolve into Los Amigos in the 1980s.