In 1959 he discovered the compositional styles of the Second Viennese School and the European avant-garde movement, and integrated both forms into his new style. The first piece to come to the attention of the musical world outside of Czechoslovakia was his 3rd string quartet (1963), in large part due to the interpretation of the piece by the Novák Quartet which performed it in its concerts throughout Europe.
In the 1960s, Kopelent became well known in contemporary European music circles, with his compositions being performed at such festivals as the Warsaw Autumn, Donaueschinger Musiktage, Witten music festival and the annual festival in Darmstadt. During this time, Kopelent also served as a member of several compositional competition juries.
From 1965 to 1973, Kopelent served as an artistic director of the contemporary music ensemble "Musica Viva Pragensis", which was conducted by his colleague Zbyněk Vostřák, and for which he wrote several chamber pieces. In the Prague musical life of the 1960s, both the ensemble and the group of composers associated with it rose in importance, developing into the Prague Group of New Music, which brought together composers, musicologists and interpreters.
In 1969 Kopelent accepted a scholarship from the Deutsche Akademie, which included a one-year artistic internship (Berliner Künstlerprogram) in West Berlin. After finishing this internship, he returned to Czechoslovakia. With the beginning of political "normalisation" following the Prague Spring, Kopelent lost his job as editor of musical scores for the publishing house Supraphon, and his music was banned by the Czech government for twenty years. He had difficulty finding another job, and was ostracized by the new Union of Composers. His ensemble Musica viva Pragensis was not allowed by the authorities to pursue its concert activity.
In 1976 Kopelent accepted a job as a piano accompanyist for children's dance schools, and this became his main occupation for the next 15 years. During the 1970s he composed many pieces, a number of them for foreign commissions, but, as he could not leave Czechoslovakia, he was unable to hear their performances.
After the Velvet Revolution in 1989, Kopelent became a music advisor in the office of president Václav Havel, and in 1991 he was named a professor of composition at the musical faculty of Academy of Fine Arts in Prague, a position he retains to this day. He was a chairman of the Czech Section of the International Society for Contemporary Music and is still chairman of the Atelier 90 composers' association.
He is the organiser and regular lecturer to International Composers' Summer Courses, held in Český Krumlov. Among his students were Czech composer Lenka Kiliç, recipient of a stabat mater at the national competition of young composers, Czech composer Markéta Dvořáková, First Prize in the 1993 national competition of young composers, Ukrainian composer Svitlana Azarova, and Latvian composer Eriks Esenvalds.
In 1991, Kopelent was honoured by the French government, which named him a Chevalier des arts et des lettres. He received the Czech Classic Award in 1999, the Herder Prize in 2001, and a Czech State Award for his lifelong contribution to Czech music in 2003.
Kopelent's works have appeared in a number of compilations of Czech composers.
- Music For Five, 1964
- Quartet for Strings no 4, 1967
- Quintet for Brass, 1972
- Ballade for Piano, 1976
- Morning Eulogy, 1978
- Toccata for Viola and Piano (1978)
- Concertino for English horn & chamber ensemble, 1984
- Mon Amour, 1988
- Karrak, 1991
- Requiem of Reconciliation (Judex Ergo), 1995