Gregory O'Neil McCarthy (born October 30, 1968) is an American former Major League Baseball relief pitcher. He played for the Seattle Mariners from 1996 to 1998.
In 2003, McCarthy pitched for the independent Macon Peaches of the Southeastern League. In 2004, he pitched for the independent Atlantic City Surf of the Atlantic League and independent New Haven County Cutters of the Northeast League.
Since his playing career ended, McCarthy has coached in the Netherlands and in baseball clinics and academies. On February 17, 2009, he was hired to be the head coach of the Mosquito Athletics Attnang-Puchheim in the Austrian Baseball League. In 2010, McCarthy played and coached for AVG Draci Brno, which plays in the Czech Extraliga.
McCarthy has the same birthday as fellow former Calgary Cannon, Canton–Akron Indian, and Seattle Mariner Erik Plantenberg.
Greg McCarthy graduated from Central High School in Bridgeport, Connecticut