Juan Oncina began his vocal studies in Barcelona with Mercedes Capsir, and later in Milan, with Augusta Oltrabella.
The same year he made his Italian debut in Bologna, as Almaviva in Il barbiere di Siviglia.
He appeared in 1949, as Paolino in Il matrimonio segreto, in Paris, and in Cherubini's L'osteria portoghese and Lully's Armide, in Florence.
The turning point in his career came in 1952, when he made his debut at the Glyndebourne Festival, where he was to appear until 1961, especially in Rossini roles such as Almaviva, Lindoro in L'italiana in Algeri, Ramiro in La Cenerentola, and most notably as Le comte Ory, possibly his greatest success.
In the 1960s, he began, perhaps unwisely, to expand his repertory towards heavier roles in opera by Verdi and Puccini.