Leonel Azevedo had prominence as a composer of classics disputed by some of the best interpreters of the Golden Age of Brazilian song. Since his early childhood, Azevedo showed interest for music, and a female cousin introduced him to the piano. At 16 he studied the first grade of theory, but abandoned the course. Learning to play the violão (acoustic guitar) by himself, Azevedo was presented with the instrument by an uncle. He opened in the radio in a show hosted by his friend Nilton Meireles. Dario Murce (Renato Murce's brother) invited him for Renato's show Horas do Outro Mundo, presented twice a week. Becoming a regular, Azevedo was sometimes accompanied by Ary Barroso. He also performed regularly at the Programa Casé, together with Sílvio Caldas, Francisco Alves, Moacir Bueno Rocha, Castro Barbosa, Luís Barbosa, Noel Rosa and others.
Azevedo was introduced to his future partner J. Cascata in 1935. Both performed in the Hora Sertaneja, writing songs to sing there. Orlando Silva recorded "Mágoas de Caboclo" and "História Joanina" by the duo, opening a series of hits. In the next year, Silva recorded the duo's samba "Juramento Falso" and the valse "Lábios Que Beijei."
With Sá Roris, Azevedo wrote the choro "Apanhei um Resfriado" (recorded by Almirante), the samba "Eu Vou Dizer" (recorded by Odete Amaral), and the valse "Maria Fulô" (recorded by Gastão Formenti, his long-time idol). With Luís Bittencourt, he wrote "Lua Triste," recorded by Sílvio Caldas. The march "Não Pago o Bonde" (with J. Cascata), recorded by Odete Amaral, was a big hit in the Carnival of 1938. "Quem Foi" (with J. Cascata) was recorded by Carlos Galhardo in 1939. With the same partner, Azevedo wrote the samba "Símbolo Sagrado" and the valse "Quero Voltar Aos Braços Teus" in 1940. With Sá Roris, Azevedo wrote a satire about the Nazi troops, "Passo de Avestruz," recorded for the Carnival of 1942 by Almirante. Orlando Silva recorded several compositions by Azevedo/Cascata in the following years: in 1945, "Ela Vai Voltar"; in 1952, the valse "Pedras Dispersas"; in 1953, the samba "Escravo do Amor"; in 1954, "De Que Vale a Vida Sem Amor." Carlos Augusto recorded an album for Sinter dedicated to the duo's compositions, Músicas de J. Cascata and Leonel Azevedo.
In the '60s, with the demise of his main partner, Azevedo only retook the compositional process in 1962, writing the samba-canção "Só" (recorded by Dorinha Freitas) with Adelmo Lima, "Vamos Sambar" (recorded by Dilermando Pinheiro) with Nelson Fonseca, and the samba-canção "Sonho Desfeito" (recorded as a tango by Dorinha Freitas) with Adelmo Lima. He also was recorded with success in that decade by Sílvio Caldas and Orlando Silva. A series of five LPs was released with his unpublished songs, Histórias de Amor.