Contreras turned professional in 1981 and lost his first title fight to WBA super flyweight title holder Khaosai Galaxy by KO in 1986. He later moved up in weight and in 1990 captured the WBO bantamweight title with a TKO over Ray Minus. Contreras vacated the title to fight Luisito Espinosa for WBA belt in 1991 and won by KO in the 5th. He lost the belt in his first defense to Eddie Cook. He retired in 1995 after a TKO loss to Johnny Vasquez. Contreras collapsed in the hospital after the bout, but recovered.