Before becoming famous, he had been performing across Puerto Rico's western coast with various musical trios, particularly with the Trio Los Duendes group. He was regarded as having a powerful voice, but was too shy to occupy a solo spot in any of the groups he performed with. After he returned from a stint at the Army National Guard in North Carolina he was discovered by a Cuban television promoter, Gaspar Pumarejo, who immediately introduced him to the Puerto Rican television world. His career formally started during the week of 23 May - 28 May, 1961, when he was featured daily in Pumarejo's show. His first formal live performance was opening for Myrta Silva at a Puerto Rican music review regularly staged at the Tropicoro Room of El San Juan Hotel in Carolina, Puerto Rico now the Wyndham El San Juan Hotel & Casino. He took voice lessons from Puerto Rican soprano Rina de Toledo.
Avellanet reached teen-idol status in Puerto Rico during the 1960s, as a member of the nueva ola music movement. Helped by Alfred D. Herger, he was a member of a group of teen sensations that included Lissette Alvarez (to whom Avellanet got married), Lucecita Benitez, Yolandita Monge, Ednita Nazario and Charlie Robles, among others. His popularity was such that he managed to do three shows on the New Year's Eve and New Year's Day of 1962. With la nueva ola, he recorded his first album, named Fugitiva.
Avellanet later participated in a group of singers named La Juventud Canta. Although not a conventional music group, La Juventud Canta toured all over Latin America, and Avellanet extended his teen-idol status to other countries such as Mexico, Argentina and Venezuela, where Avellanet enjoyed a solid teen girl fan base.
Avellanet would soon join the United Artists record label, making what would be his only attempt at a cross-over into the American pop genre, with the album In a Little Spanish Town. His fourteen other records under that label were recorded in Spanish.
1972 was a very eventful year for Avellanet, who finished in fourth place at that year's Festival OTI in Spain. Then, Avellanet made history by becoming the first Hispanic to perform at the Miss Universe contest, held that year in the Puerto Rican beach town of Dorado.
Chucho Avellanet diversified, and by the 1980s, he was as known for his acting as he was for his singing. Having signed up with Canal 2, he was able to explore his ability both in the show hosting and acting fields when he hosted, alongside Lissette Alvarez, the popular El show de Chucho y Lissette. He also hosted Senoras y Senores and Con Chucho Avellanet before finally settling with El Show de Chucho. Avellanet also made comedy at El Show de Chucho, participating at the show's comedy section. In one particularly memorable comic chapter, he screamed, May the roof fall on top of us if Im lying! and the roof then 'came down' on him and his guests. He also participated, with Lissette, in a famous Schaeffer Beer commercial, where he sang the jingle Cerveza Schaeffer es la mejor cuando se toma mas de una! (Schaeffer Beer is the best when you drink more than one!!). During the filming of the commercial the vessel they were at was rocked by a submarine of the United States Navy navigating below it; the experience left him with a lasting impression, which years later motivated him to support the cause of the people of Vieques in their demands to have the Navy vacate their bombing range surrounding Vieques and Culebra.
One of Avellanet's biggest international hits during the decade of the 80's came in his song Sera varon, sera mujer? (Will it be a boy, will it be a girl?), where he explored the questions that couples expecting a baby might go through. This song coincided with his wife's pregnancy, and in his case, his baby turned out to be a boy. He was by then married to his second wife, famous Venezuelan actress Marisela Berti, whom he later also divorced.
In 1988, Avellanet came back to the headlines, albeit not precisely because of his own, personal success: His nephew Roberto Alejandro Avellanet Negrón was chosen as the newest member of Menudo. Roberto followed in his uncle's footsteps, becoming a major teen-idol in Puerto Rico within months of his joining the teenaged boy band.
In 1994, he, Lucecita Benitez and Lissette Alvarez rejoined forces for a one-night only concert.