After intentionally pitching poorly during the 1994-95 Cuban National Series, Prieto was allowed a visa to leave Cuba in April 1995. He was selected with the fifth overall selection in the 1995 MLB Draft by the Oakland Athletics. Prieto made his major league debut for the Athletics that July. Prieto, being unfamiliar with American banks and credit cards walked around with his $1.2 million signing bonus in cash in his pocket for over a week. He won two games and lost six, becoming one of just a handful of players who were drafted and then played in the big leagues during the same season. In 1996 Prieto had what was arguably his best season, winning 6 games and losing 7 with an ERA of 4.15.
Prieto was traded to the Tampa Bay Devil Rays before the 2001 season. With the Devil Rays, Prieto saw action in three games, without any decisions, allowing one earned run in 3.2 innings pitched. His one season with the Devil Rays was his last season in the major leagues. Prieto surfaced in Mexico during the middle 2000s, playing with the Venados de Mazatlán, a team with which he reappeared at the Caribbean World Series, held that year in Venezuela. During six MLB seasons, Prieto won 15 games and lost 24, with a 4.85 ERA.
He played until 2005 in the minor leagues, unable to make it back to the A's.
Prieto spent the 2009 through 2011 seasons as the pitching coach for the Athletics' Arizona League team.
On November 10, 2011, Prieto was announced as the pitching coach for the Vermont Lake Monsters, the Athletics' New York-Penn League (Single A, short season) team.
Since the 2012 season, Prieto has served as interpreter for fellow Cuban defector, Yoenis Cespedes, who plays for the Athletics.