Tosh was born in Westmoreland, the westernmost parish of Jamaica. He was abandoned by his parents and “shuffled among relatives.” When McIntosh was fifteen, his aunt died and he moved to Trenchtown in Kingston, Jamaica. He first picked up a guitar by watching a man in the country play a song that captivated him. He watched the man play the same song for half a day, memorizing everything his fingers were doing. He then picked up the guitar and played the song back to the man. The man then asked McIntosh who had taught him to play; McIntosh told him that he had. During the early 1960s Tosh met Robert Nesta Marley (Bob Marley) and Neville O’Reilly Livingston (Bunny Wailer) and went to vocal teacher Joe Higgs, who gave out free vocal lessons to young people, in hopes to form a new band. He then changed his name to become Peter Tosh and the trio started singing together in 1962. Higgs taught the trio to harmonize and while developing their music, they would often play on the street corners of Trenchtown.
In 1964 Tosh helped organize the band the Wailing Wailers, with Junior Braithwaite, a falsetto singer, and backup singers Beverley Kelso and Cherry Smith. Initially, Tosh was the only one in the group who could play musical instruments. According to Bunny Wailer, Tosh was critical to the band because he was a self-taught guitarist and keyboardist, and thus became an inspiration for the other band members to learn to play. The Wailing Wailers had a major ska hit with their first single, “Simmer Down”, and recorded several more successful singles before Braithwaite, Kelso and Smith left the band in late 1965. Marley spent much of 1966 in Delaware in the United States with his mother, Cedella (Malcolm) Marley-Booker, and for a brief time was working at a nearby Chrysler factory. He then returned to Jamaica in early 1967 with a renewed interest in music and a new spirituality. Tosh and Bunny were already Rastafarians when Marley returned from the US, and the three became very involved with the Rastafari faith. Soon afterwards, they renamed the musical group the Wailers. Tosh would explain later that they chose the name Wailers because to “wail” means to mourn or to, as he put it, “…express one’s feelings vocally”. He also claims that he was the beginning of the group, and that it was he who first taught Bob Marley the guitar. The latter claim may very well be true, for according to Bunny Wailer, the early Wailers learned to play instruments from Tosh.