Antonio Mauricio Horta de Melo was born in Belo Horizonte on December 2, 1948, into a family that had music in it three generations deep. His grandfather, Maestro Joao Horta, wrote sacred and popular music in Minas Gerais. His mother Geralda played the bandolim, the Brazilian mandolin, and gave him his first lessons. His father played violin. His older brother Paulo worked as a professional contrabassist and brought home the American jazz records of the 1950s and 1960s that shaped what Toninho would later do with a chord. His first documented appearance in the recording industry is not as a player at all: the 1964 Polydor LP O Melhor da Noite, by the vibraphonist Aecio Flavio and his group, carries a song on its fourth track called "Flor Que Cheira a Saudade," credited to Toninho Horta and Gilda Horta, his sister. He was fifteen. His brother Paulo is the contrabassist on the same record.

He surfaced publicly as a composer at the II Festival Internacional da Cancao in Rio in 1967, and he met Milton Nascimento when he was fourteen, the friendship that organized the next fifty years of his working life. His first credit on a Nascimento record is precise and small: on Nascimento's self-titled 1969 Odeon album (MOFB 3592) he appears on exactly one track, "Aqui, Oh!", where he sings and is the composer and plays nothing. He moved to Rio de Janeiro in 1970, joined Elis Regina's band, and fell in with A Tribo, the short-lived group he shared with Joyce, Nelson Angelo, Novelli and Nana Vasconcelos, which cut the Odeon single Tapinha in 1970 and contributed to the Odeon various-artists LP Posicoes.

What made his name was Clube da Esquina, the 1972 Odeon double album credited to Milton Nascimento and Lo Borges. Horta has no album-level credit on it at all. He has sixteen track-level credits, spread across all four sides, on five different instruments: electric guitar, acoustic guitar, bass, percussion and backing vocals. On side one he plays the electric guitar solo on "O Trem Azul." That scattered, utility-player shape is exactly what a Clube da Esquina session was, and it is why his role in the record is so often described vaguely. Over the decade that followed he did the same job on a run of Nascimento albums, and on each of them the credit that matters is one he wrote himself: on Minas (EMI, 1975) he plays acoustic guitar and piano, sings, arranges, and is the composer of "Beijo Partido"; on Geraes (EMI, 1976) he plays electric piano and organ, orchestrates, and wrote "Viver de Amor"; on Clube da Esquina 2 (EMI, 1978) he plays acoustic guitar, electric guitar and piano, sings, orchestrates and conducts, and wrote "Dona Olimpia (Your Moon)."

One session in that stretch changed what happened to him afterward. In January 1977 A&M issued Milton (SP-4611), recorded at Shangri-La Studios in Malibu and The Village Recorder in Los Angeles, with a band of Milton Nascimento, Toninho Horta on twelve-string and electric guitar, Wayne Shorter on soprano and tenor saxophone, Herbie Hancock on piano, Raul de Souza on trombone, Hugo Fattoruso on piano and organ, Novelli on bass and Robertinho Silva on drums. The Wayne Shorter and Herbie Hancock associations that turn up in every biography of Horta come from that one album, and the sources that mention them rarely say so. American work followed on the same footing: guitar across most of Airto Moreira's Promises of the Sun (Arista, 1976), a single track on Flora Purim's Nothing Will Be As It Was... Tomorrow (Warner Bros., 1977), and three tracks on The Manhattan Transfer's Brasil (Atlantic, 1987).

His own leader catalog started late and then went abroad. Terra dos Passaros, recorded piecemeal between July 1976 and September 1979 and issued by EMI in 1980, and the self-titled Toninho Horta on EMI-Odeon the same year, are the two Brazilian records his reputation at home rests on. The second of them carries a line in Portuguese on the sleeve reading that Pat Metheny appears courtesy of ECM Records; Metheny plays guitar on two of its tracks. That connection carried Horta into the American jazz industry. Verve Forecast, PolyGram's jazz imprint, released Diamond Land in 1988 with liner notes by Metheny, Moonstone in 1989 with Metheny playing on it alongside Randy Brecker, Eliane Elias, Mark Egan, Danny Gottlieb and Nana Vasconcelos, and Once I Loved in 1992, a trio record with Gary Peacock on acoustic bass and Billy Higgins on drums, produced by Eliane Elias. He moved to New York around the end of the 1980s and stayed for most of the decade, cutting Durango Kid (1993) and Durango Kid 2 (1995) for the American independent Big World and Foot on the Road (1994) for PolyGram.

The later work went back to Brazil and to Japan. From Ton to Tom, his Antonio Carlos Jobim tribute, was recorded in Belo Horizonte and Rio in 1996 and 1998 and at Avatar Studios in New York in April, June and July 1998, with Gary Peacock, Bob Mintzer, David Kikoski and Gal Costa among the players; it first appeared in Spain in 1999, in Brazil and Japan in 2000, and was reissued by Resonance in the United States in 2008 retitled To Jobim With Love. He founded his own Minas Records, made duo and partnership records with the Italian guitarists Antonio Onorato and Stefano Silvestri, with the flautist Nicola Stilo, with Joyce, with Alaide Costa and with the Polish singer Dorota Miskiewicz, and in 2019 released the double album Belo Horizonte with his Orquestra Fantasma, one disc of older songs and one of new ones, marking fifty years of work. It won the 2020 Latin Grammy for Melhor Album de Musica Popular Brasileira (Best MPB Album), beating finalists including Caetano Veloso, Ney Matogrosso, Zeca Baleiro and Elza Soares.

Horta's subject is the chord, not the line. He told interviewers he found most of his harmonic vocabulary by himself, moving voicings around on the fingerboard rather than working from theory, and the result is a set of extended and altered shapes that belong to post-bop harmony rather than to the standard bossa nova grammar, laid over Minas Gerais melody and a guitar tone that is soft and unhurried even at speed. He came up hearing Jobim, Baden Powell and Joao Gilberto at home and Stan Getz, Chet Baker, Ella Fitzgerald and the Beatles with his friends, and he has said he picked up the electric guitar at fifteen after hearing Wes Montgomery, Barney Kessel and Tal Farlow. Pat Metheny, who wrote the liner notes for Diamond Land and played on two of Horta's albums, has called him one of the world's great composers on nylon-string guitar and, in the line most often quoted, "the Herbie Hancock of bossa nova guitarists." His songs travel further than his records do: Claudio Roditi cut one of them on Bons Amigos in 2011, Duduka Da Fonseca's trio devoted a whole 2011 Zoho album, Plays Toninho Horta, to his compositions, and "Beijo Partido," "Aquelas Coisas Todas" and "Diana" have become standards among Brazilian and American players alike. Rolling Stone Brasil placed him among the thirty greatest Brazilian guitar icons in 2012, and Legacy/Columbia included him in the 2005 anthology Progressions: 100 Years of Jazz Guitar.