Rojas was born and raised in Chile. He began his career in 1976 with the Chilean club Aviación. Rojas went on to play for Colo-Colo from 1983 until 1987. With Colo-Colo, Rojas won national titles in 1983 and 1986. In 1987, after a successful performance in the Copa América, he transferred to Brazil's São Paulo where he remained until 1989. In 1987 Michel Preud'homme, considered the best goalkeeper in the world by the FIFA, declared for German magazine 11freunde that: "Roberto Rojas is better than me". After his retirement he returned to São Paulo to serve as a goalkeeper coach, training Rogério Ceni. In 2003, Rojas served as interim coach and took the team to the Copa Libertadores for the first time since 1994. He was later a goalkeeping coach for Brazilian side Sport Club do Recife.