He wrote scores for more than 70 feature films and documentaries, including Finding Neverland (2004), for which he won an Oscar and a National Board of Review Award.

Other notable scores were for Hachi: A Dog's Tale, Unfaithful, Evening, The Visitor, and Washington Square.

[1] His music has been released by Sony Classical, Decca, Varèse Sarabande, Verve, Epic, Milan, and Savitor Records.

[1] In 2005, Kaczmarek received the Academy Award for Best Original Score for Finding Neverland, directed by Marc Forster, on which he had worked with music editor Christopher Kennedy, among others.

[1][5] In addition to his work in films, Kaczmarek was commissioned to write two symphonic and choral pieces for two important national occasions in Poland: Cantata for Freedom (2005) to celebrate the 25th anniversary of the Solidarity movement, and Oratorio 1956 (2006) to commemorate the 50th anniversary of a bloody uprising against totalitarian government in Poznań, Poland.

Jan's last monumental work, Emigra – The Neverending Symphony, was performed in February 2017 in Gdynia, Poland.