Contreras played for Pinar del Río Vegueros of the CNS, and competed for the Cuban national team in the Summer Olympics, the Pan American Games, the Baseball World Cup, and the 1999 exhibition series against the Baltimore Orioles.

He also played in MLB for the Chicago White Sox (2004–2009), Colorado Rockies (2009), Philadelphia Phillies (2010–2012), and Pittsburgh Pirates (2013).

[citation needed] In March 1999, Contreras pitched for the Cuban national baseball team against the Baltimore Orioles in an exhibition series at Estadio Latinoamericano in Havana.

[3] "After the Orioles game, [Contreras] was the man in Cuba," said Joe Kehoskie, at the time an agent for Cuban defectors.

[4] Contreras defected from Cuba in October 2002 while pitching for the Cuban national team during the Americas Series in Saltillo, Coahuila, Mexico.

[5] Contreras's defection set off a media frenzy, as he had been considered a loyalist and had shown no prior interest in leaving for MLB.

[7] Contreras went 7–2 and posted a 3.30 ERA in 18 games (nine starts), but he spent two months on the disabled list with a subscapularis strain,[8][9] had four stops in the minor leagues, and was in and out of the bullpen.

He struck out ten batters, but what was more important to him is that for the first time since being in Cuba, Contreras got to pitch in front of his ex-wife, Miriam and daughters, Naylan and Naylenis.

Contreras started the season with five consecutive no-decisions and an ERA of 3.04, but pitched poorly after that until the All-Star break, with a 4–3 record.

[24] Dating back to the 2005 season, Contreras won 17 consecutive games before finally taking the loss against his former team, the New York Yankees on July 14, 2006.

[28] Contreras struggled mightily in the 2007 campaign along with the disappointing White Sox team who finished fourth place in the American League Central Division.

[10] One positive note for Contreras were the career-high two shutouts recorded on May 10 against the Minnesota Twins and on September 19 against the Kansas City Royals.

[29][30] Contreras ruptured his Achilles Tendon on August 9, 2008, while attempting to field a ground ball against the Boston Red Sox.

Notching his second consecutive victory, he threw eight innings of shutout baseball once again, giving up two hits and two walks while striking out eight.

[37] On August 31, 2009, Contreras was traded to the Colorado Rockies with cash for minor league pitcher Brandon Hynick.

He recorded his first career save on May 15 against the Milwaukee Brewers after taking over the closer's role from the injured Brad Lidge.

[47][48] The team granted the pitcher his release on March 22, 2014, when the club informed Contreras that he would not make the Opening Day roster.

[49] On April 1, 2014, Contreras signed with the Toros de Tijuana of the Mexican League in 2014 he made 23 starts across 134 innings going 10-3 with a 3.49 ERA and 140 strikeouts.