Guilherme Monteiro was born in Rio de Janeiro on July 20, 1971, during the years of Brazil's military dictatorship. He began pestering a neighbor's music teacher for lessons at seven, and the family soon relocated to Belo Horizonte, where he grew up. By fifteen he was performing in the city's jazz clubs and teaching privately, and as a teenager he played alongside Toninho Horta and Tavinho Moura, absorbing the dense harmonies and shifting rhythms of the Minas Gerais school of Brazilian guitar that both men helped define.

In 1995 he moved to the United States, working R&B and jazz rooms in Minneapolis and then Boston, where he attended Berklee College of Music. In 1999 he was accepted into the Thelonious Monk Institute of Jazz Performance, a highly selective program that auditions musicians worldwide and admits only eight players per instrument. There he studied privately with Clark Terry and Jimmy Heath, performing and teaching clinics alongside both men. He settled in New York City in 2000 and, after a rough initial stretch, began working steadily with some of the city's most established jazz musicians: he appears on Harry Allen's If Ever You Were Mine (2003, BVCJ-34023) alongside Ron Carter and Grady Tate, and on trombonist Slide Hampton's Jobim tribute Slide Plays Jobim (2002, Alleycat Records), issued on bassist John Lee's own imprint with Claudio Roditi and Duduka Da Fonseca. He also recorded and performed with Kenny Werner, Lee Konitz, Randy Brecker, Bernard Purdie and the Brazilian singer Johnny Alf, among others. In September 2007 he shared a duo bill with guitarist Lionel Loueke at the 3rd International Guitar Festival in Belo Horizonte.

Monteiro's guitar has held the chair a piano would normally occupy in several notable large-ensemble and small-group settings. He plays guitar on Anat Cohen & The Anzic Orchestra's Noir (2007, Anzic 1201), the ten-piece date that substituted his guitar for a piano and three cellos for violins; a contemporary All About Jazz review singled out his playing by name among the album's featured soloists. He returned alongside Cohen twice more: on drummer Duduka Da Fonseca's Samba Jazz - Jazz Samba (2012, Anzic ANZ-0041), where he again holds the guitar chair in Da Fonseca's quintet, and on bassist Nilson Matta's Black Orpheus (2013, Motéma), a reimagining of the 1959 film score with Cohen on clarinet and Randy Brecker on trumpet. He was also a featured guest, alongside Eliane Elias, Michael Brecker and bassist Marc Johnson, on Elias's Dreamer (2004); appears on vocalist Luciana Souza's Duos II (2005, Sunnyside), which earned Souza a Grammy nomination for Best Jazz Vocal Album at the 48th Annual Grammy Awards; and was a featured soloist, alongside clarinetist Paquito D'Rivera, on Arturo O'Farrill and the Afro-Latin Jazz Orchestra's 40 Acres and a Burro (2011, Zoho), a Grammy nominee for Best Large Jazz Ensemble Album. He also recorded and toured Europe and Lebanon with Ron Carter's group in the summer of 2009, and appears on Carter's Blue Note release Jazz & Bossa (2008).

Alongside his jazz sideman work, Monteiro spent four years touring and recording with the singer Lila Downs, whose Una Sangre = One Blood (2004) won a Latin Grammy for Best Folk Album, and he has been a core member since 2002 of Forró in the Dark, a New York-based collective that reworks Brazilian forró with rock, folk and jazz elements; the band's debut, Bonfires of São João, featured guest appearances from David Byrne, Miho Hatori and Bebel Gilberto. He released his own leader debut, Air (2009), on Brooklyn Jazz Underground Records, and later co-founded the experimental project Green Lotus. In 2019 he joined saxophonist Michael Blake's Chroma Nova, appearing on Dance of the Mystic Bliss (2023, P&M Records), and in 2025 he played guitar on drummer Daniel Freedman's Saha Gnawa (Pique Nique), a collaboration with the Moroccan maalem Hassan Ben Jaafar.

Monteiro's playing carries the harmonic vocabulary of the Minas Gerais school he absorbed as a teenager under Toninho Horta into a New York jazz context, and his private study with Clark Terry and Jimmy Heath at the Thelonious Monk Institute shows in the clarity and swing of his comping. As the guitarist called on to replace a piano chair, on The Anzic Orchestra's Noir and in Duduka Da Fonseca's quintet, he favors a warm, unhurried single-line sound over dense block chords, and reviewers of Noir singled him out by name alongside the record's horn soloists. His catalog splits cleanly along two tracks that rarely overlap: disciplined, harmonically sophisticated jazz sideman work for leaders like Ron Carter, Harry Allen and Anat Cohen, and the looser, groove-driven Brazilian roots music of Forró in the Dark and his work with Lila Downs, where the emphasis shifts from improvisation to rhythm and song.