Jacques Pellen was born in Brest on April 9, 1957, and lived in Portsall, a coastal village in Ploudalmézeau, at the western tip of Brittany. He began classical guitar at twelve. The Breton revival of the 1970s, led by Alan Stivell and the guitarist Dan Ar Braz, drew him toward Celtic music, and at the Lycée de Kerichen in Brest he listened to Keith Jarrett, John McLaughlin and Pat Metheny. The Breton music magazine Rythmes Croisés adds that he jammed with the Irish musician Donal Lunny at the Brest youth centre in 1973, and that Jan Garbarek, Metheny and McLaughlin sat beside Messiaen and Schoenberg among his references. He left the École normale in Quimper to become a professional musician and first played in Brest groups, among them Gwalarn, whose 1976 album on Disques Vélia lists him on guitar, and Bleizi Ruz, a fest-noz band. In 1978 he played on a record by the singer Melaine Favennec.

His turn toward jazz came in the mid-1980s. In 1986 he formed a trio with Gildas Scouarnec and the Hungarian-born drummer Peter Gritz, and Gritz introduced him to the trumpeter Kenny Wheeler, who had just recorded with Keith Jarrett. The result was Jacques Pellen 4 (Caravan, 1989), made in December 1988 at Studio Amadeus with Gritz, Wheeler and the double bassist Riccardo Del Fra. The music was created for evenings called Jazz à Vauban; it includes a reworking of a Scottish air, "I Can Get No Sleep," and a piece titled "Impression de Coltrane." Wheeler and Del Fra remained the core of his jazz circle for years.

Celtic Procession, the project for which he is best known, grew out of an earlier "Celtic Connection" idea dated 1987 and was formed in 1988. It was never a fixed band: Rythmes Croisés reports that Wheeler appeared on the first album, Celtic Procession (Silex, 1993, recorded in February 1993 at the GAM studio in Waimes, Belgium), that Paolo Fresu joined from 1994, and that stand-ins such as the saxophonist Eric Barret, the bassist Gildas Boclé and the violinist Jacky Molard, with his brother Patrick, filled the chairs when regulars were away. Pellen himself described the music as a jazz aesthetic under Breton influence, and the lineup ranged from a quartet to a large ensemble that included Breton singers such as Erik Marchand and Annie Ebrel. In the same year he made Triptyque (Gwerz Pladenn, 1993) with the Molard brothers, a trio record of traditional tunes, original pieces and his own arrangements, and he took part in Dan Ar Braz's Héritage des Celtes project, which had a considerable success in France.

The records of the late 1990s and 2000s tie him closer to the jazz labels. Sorserez (Gwerz Pladenn, 1996) is a guitar and double bass duo with Del Fra. Condaghes (Silex, 1998) pairs him with Fresu and the Breton singer Erik Marchand, with Henri Texier on double bass on the Discogs release record. In 1998 he invited the violinist Didier Lockwood to a Celtic Procession concert at the Festival Interceltique de Lorient, and the group closed the 20th Les Tombées de la Nuit festival in Rennes on July 10, 1999, a concert issued as Les Tombées de la Nuit: A Celtic Procession Live (Naîve, 1999) with Fresu, Dan Ar Braz, Marchand and the harpist Kristen Noguès. Éphéméra (Naîve, 2003, recorded in 2001) reunited him with Del Fra, Gritz and Fresu and added the keyboardist Patrick Péron, with Marchand and Ebrel as guests, and closes on Bob Dylan's "Mr. Tambourine Man." Noguès, his partner, died in 2007; that year he released Lament for the Children (Naîve) with Gildas Boclé and the drummer Marcello Pellitteri, a trio record of his own pieces plus "God Bless the Child."

In 2011 he founded the trio Offshore with the bassist Étienne Callac and the drummer Karim Ziad, a crossover of Celtic music and North African rhythms whose first album, Off Shore, came out on Boutou Production and Coop Breizh. That year he also launched Les Séries, a project with Annie Ebrel and the jazz-rock quartet One Shot built on unreleased material by Noguès. Quiet Place (L'Autre Distribution, 2013) is a duo of guitar and saxophone with Eric Barret, whom he called an ally of thirty years. Morenn (2014) is a bagpipe and guitar record with Xavier Bodériou and the flautist Sylvain Barou, and Barou then joined Offshore, which became a quartet for Shorewards (Paker Production, 2017). From 2015 he returned to the stage and studio with Dan Ar Braz, and in 2017 he joined Ar Braz's string trio Guitares with David Er Porh, which toured until 2020. His only solo album, A-hed an Aber (Paker Production, 2018), mixes his own pieces with Gershwin's "I Loves You Porgy" and Henri Texier's "Don't Buy Ivory, Anymore." He had also taken up painting, in watercolor and, from 2011, in oils, and began exhibiting in 2014.

In April 2020 Pellen tested positive for the coronavirus and spent three weeks in hospital in Brest. He passed away there on April 21, 2020, at the age of 63, of complications of COVID-19. Franck Bergerot wrote in Jazz Magazine on the day of his death that he was a major and under-recognized figure of French jazz, a link between Breton traditional music and jazz writing and improvisation. The Offshore quartet's last album, Standing on the Shore, recorded between 2016 and 2020, was released on Paker Prod in February 2021.

Pellen played acoustic and twelve-string guitar, with guitar synthesizer on some 1990s records, and wrote in a spare, open manner that he linked to the cool European sound of ECM and Jan Garbarek rather than to Afro-Cuban or hard-swinging jazz; in the Rythmes Croisés interview he said he found jigs and reels on guitar unrewarding and preferred jazz harmony, while keeping what his folk training had given him. Breton melodies, modal tunes and Scottish airs run through his work, set against improvisation by a rotating cast of jazz players: Wheeler, Fresu and Barret on trumpet and saxophone, Del Fra and Texier on double bass. Because the music sits between the two traditions, catalogues file it both ways. English Wikipedia and Wikidata call him a jazz guitarist and Jazz Magazine's obituary treats him as a major figure of French jazz, while AllMusic lists his styles as Celtic, French pop and Breton, and several of his records, from Triptyque to Morenn, are Breton folk projects. The Subgenres tags above reflect the jazz side of that mix.