Braithwaite was with The Wailers for eight months and sang lead on such songs as "Habits", "Straight and Narrow Way", "Don't Ever Leave Me", and "It Hurts To Be Alone". He had the best voice in The Wailers, according to Studio One's Coxsone Dodd, who discovered the band's talent. Bob Marley later commented: "Junior used to sing high. It's just nowadays that I'm beginning to realize that he sounded like one of the Jackson Five. When he left we had to look for a sound that Bunny, Peter and me could manage."
Braithwaite left the band in 1964 and moved to the United States with hopes of pursuing a medical career. He lived primarily in Chicago and southern Wisconsin for the next 20 years, and returned to Jamaica in 1984 to work with Bunny Wailer on a Wailers' reunion project. With the assassination of Peter Tosh in September 1987, plans for world tours with a reunited Wailers never materialized.
Braithwaite was murdered on 2 June 1999 in the home of a fellow musician in Kingston.