Rosario was born and raised in Coamo, Puerto Rico into a poor, but hard-working family. His parents realized that as a child Willie was musically inclined and had him take guitar lessons at the age of 6. He received his primary and secondary education in his hometown. His mother also had him take saxophone classes; however, what he really was interested in was the conga. In 1946, he formed a band called Coamex but, a year later when he was 17 years old, his family moved to New York City.
The family rented an apartment in the Manhattan community known as Spanish Harlem. Rosario came into contact and played the conga for various orchestras. After he graduated from high school, he continued his education and studied journalism and public relations. On one occasion Rosario visited the famed Palladium Ballroom in New York, where Tito Puente was playing the timbales.
This was the beginning of his love affair with that instrument. He was 22 years old when he took classes with percussionist Henry Adler. Among his best-known compositions and arrangements are “De Barrio Obrero a la Quince” (De Barrio Obrero a la Quince) was actually written by Trinidad Clemente, “El Timbal de Carlitos”, “Mi Amigo el Payaso” (My friend the Clown) with lyrics by Luis Antonio Ruiz, “El Revendón”, “Lluvia” (Rain), “Cuando No Hay Cariño” (When There’s No Love). Plus, he also composed the following jazz tunes “Flip, flop”, “Stop and go” and “My Favorite Thing”.