Photo: Marianne Grøndahl

John Frandsen wrote a lot of vocal music, both solo and choral works. The most important titles within this genre are the Blake cycles Songs of Innocence (1984) and Songs of Experience (1991) for voice and guitar, Seven Silly Songs (1987) for mezzo and guitar, Winternächte (1989) for mezzo and viola, Fons Amoris (1998) for choir a cappella, and Uroligt Requiem (2019) for countertenor and sinfonietta. Some works are church-related, including Stabat Mater (1986) for tenor/soprano and organ, Magnificat (1990) for double choir and Messe til det Nye Årtusinde (1999) for soloists, choir, organ and orchestra, which was premiered in Aarhus Cathedral on New Year's Eve 1999/2000. Recent years include the motets Benedic Domino (2006) and Psalmodie (2020), both written for his own chamber choir Convivium, De Profundis (2007) and O Sacrum Convivium (2019) written for the Ars Nova vocal group, as well as the Blake cycle The Divine Zoo (2010) for the DR Girls' Choir.

In November 2010, he completed a one and a half hour long Requiem, commissioned by the DR Radio Symphony Orchestra and premiered at the Concert Hall in April 2013. The Requiem included some newly written Danish hymn texts by the author Simon Grotrian. And the same poet has been set to music in a number of other vocal works from recent years: Wheather4casts (2012) for 12 solo voices, Juleroser og Engleskyer (2013) for girls' choir and harp and Skt. Annæ Messe (2014) for soloists, girls' choir, organ and reed bells.

The singable, melodic element also plays a prominent role in the instrumental works. This is particularly true in the early works such as String Song (1980) for string quartet, the wind quintet Avers/Revers (1985) and Nature Morte (1986) for guitar solo. Many works have a dramatic and image-creating character - for example, the symphony Dæmonernes Dans (1988), the string quartet no. 2 Danse Macabre (1991) and the cello concerto Hymn to the Ice Queen (1999). Associations with well-known musical genres and styles are often played with, which sometimes gives the music an ambiguous expression. This applies, for example, to Variations on a Falling Star (1996) for chamber ensemble, which is designed as a series of variations on a well-known Disney melody, and the flute concerto Beauty and the Beast (2011), which plays with blues fragments. He has also been interested in abstract instrumental theatre, which is expressed in the wind quintet no. 2 De/cadences (1987) and in Hymne til Askefuglen (1992) for choir and band. His organ works include Partita over Nu Blitzer Åndens Hvide Lys (1999), Tombeau d'un Ange (2015) and Symphony for organ (2021).

In recent years, he has worked on a series of solo, chamber and orchestral works designed as musical portraits of Greek gods, Olympic Suite. The series of works includes the orchestral works Morpheus (2025) written for the Danish Chamber Orchestra and ZEUS (2026) for the Odense Symphony Orchestra.

As an opera composer, John Frandsen has made a significant impact. In May 1995, the Music Theatre "Undergrunden" premiered his opera Amalie based on the work of Amalie Skram. Some of the music from this is included in the Amalie suite for chamber orchestra. In 1997, his monodrama Boudicca was premiered at Den Anden Opera, and from 1998 he began work on an opera trilogy based on Svend Åge Madsen's novel Vice and Virtue in the Middle Time, a project that was realized in collaboration with Aarhus Summer Opera. Part 1 was premiered in the summer of 1998, and part 2 two years later in 2000. Part 3 had its premiere in August 2004 in connection with a combined performance of the entire trilogy. The Vice and Virtue trilogy was awarded a prize by the Danish Arts Foundation in December 2004 and nominated for the Reumert Prize in March 2005. In the summer of 2001, the church opera Undermålerne was premiered to a text by Ivar Gjørup. And on March 15, 2003, the Royal Danish Opera premiered his first major symphonic full-length opera I-K-O-N with a libretto by Sanne Bjerg and directed by Peter Langdal. In the summer of 2013, the Aarhus Summer Opera premiered his opera 1/2LELUJA based on the comic strip EGOLAND and created in collaboration with the series' author Ivar Gjørup. In November 2014, the Funen Opera premiered the chamber opera A Doll's House, written based on Henrik Ibsen's theatre classic. In January 2017, the chamber opera Martyriet, based on the fate of the poet-priest Kaj Munk, premiered on the Royal Danish Opera's Takkelloft. And in August 2021, the Aarhus Summer Opera set the stage for the premiere of the opera Dyrets År 1666, based on the author Lone Hørslev's sensual novel about the fate of the young Marie Grubbe in the doomsday winter of 1666.