A native of Bajos de Haina of the Dominican Republic, the six-foot right-hander originally signed with the Cincinnati Reds as a non-drafted free agent on 11/27/04. He spent the first two-years of his career pitching for the Reds in the Dominican Summer League, where in 2005 he went 1-0 with eight saves and a 0.38 ERA over 16 appearances striking out 29 batters over 23.2 innings of work. In 2006, he appeared in 29 games for the DSL Reds and went 0-1 with 27 saves striking out 36 batters while walking just nine with opponents hitting just .155 against him.

In 2007, Geronimo moved from pitching in the DSL to the Midwest League where he went 7-3 with a 2.45 ERA and 12 saves in 49 appearances with Class A Dayton. With the Dragons he didn't allow an earned run in any of his first 14 appearances to begin the season (17.3 IP, 1 R) and pitched 18.1 consecutive scoreless innings from June 11 to July 28, overall he recorded 39 scoreless appearances. Began the 2008 season with Class A Advanced Sarasota from April 4 - July 18 where he had 11 saves in 36 games for Reds, with a 0.87 ERA which helped earn a promotion to Double-A on July 20. His performance earned him while in Sarasota earned him selection to the FSL Mid-Season All Stars on 6/14/2008.

In the latter part of 2008, the Reds moved their Double A player development agreement (PDA) from Chattanooga to Zebulon (suburb of Raleigh) North Carolina. Last year with the Mudcats, Geronimo appeared in 50 games posting a record of 4-3 with an ERA of 4.87 along with 5 saves, in 64.2 innings pitched.