Chacín signed with the Colorado Rockies as an international free agent in 2004, and was considered to be one of their top prospects prior to 2009 and 2010.
Chacín was called up directly to the majors from the Class AATulsa Drillers on July 24, 2009. In 2010 in his first full season in MLB with the Rockies Jhoulys Chacín had nine wins, 3.28 ERA, and led all NL rookies in strikeouts with 138.
Chacín started the 2011 season as one of the best young pitchers in baseball, but after the second half of the season, he became inconsistent with his mechanics. He did, however, finish with a record of 11–14 with a 3.62 ERA in 31 starts. In 194 innings pitched, he struck out 150. However, he led the National League in walks with 87. He threw his first career shutout in his first career complete game on April 15, 2011.