Small was drafted by the Toronto Blue Jays in the 22nd round of the 1989 Major League Baseball Draft. He debuted for the Blue Jays in 1994, but was traded to the Florida Marlins in 1995 for minor leaguer Ernie Delgado. Over the next ten years, Small would split most of his time between Triple-A and the majors, with major league appearances for the Oakland Athletics, Arizona Diamondbacks, Atlanta Braves, and a second stint with the Marlins, before joining the Yankees. In addition, Small was also a member of the Milwaukee Brewers, Tampa Bay Devil Rays, Colorado Rockies and Anaheim Angels organizations, though he never pitched in the major leagues for those teams.
New York Yankees
Small began 2005 in the Yankees minor league system, pitching for Double-A Trenton and Triple-A Columbus. Due to injuries, he was called up to the Yankees to fill in, making his first start on July 20, which he won. Small recorded 10 major league victories without a loss. Small recorded his first major league complete game shutout, which came against the Oakland Athletics on September 3, 2005. His pitch count during the game was just over 110 and he struck out the last batter he faced, Dan Johnson, and Small received the baseball from catcherJorge Posada as a remembrance. Small was the first Yankees pitcher to win his first 9 decisions since Tommy John in 1979, and became just the fourth pitcher in history to win at least 10 games without a loss, joining Tom Zachary, Dennis Lamp, and Howie Krist. He finished the 2005 season 10–0 with a 3.20 earned run average and 37 strikeouts. He was re-signed to a one-year, $1.2 million contract during the off-season. During 2006 spring training, Small injured his right hamstring. He missed the start of the season, but returned to the team on April 30, 2006. On June 17, 2006, Small was designated for assignment by the Yankees and was outrighted to Triple-A Columbus.
Post-Yankees
On January 23, 2007, Small signed a minor league contract with the Seattle Mariners. However, on May 18, 2007, he announced his retirement after being released by the Mariners.
On August 2,, Small took part in the 62nd Annual Old-Timers' Day at Yankee Stadium. Six weeks prior, Small survived a bout with encephalitis that included a medically induced coma lasting eight days. He was again honored by the New York Yankees at Old Timers' Day on June 26, 2011, and July 1, 2012. Small, along with his wife Macy, is a devout Christian. He is active in his church, Fairview Baptist Tabernacle in Sweetwater, Tennessee, where they served as youth directors for 11 years. Small runs his own baseball facility in Loudon, TN call Aaron Small’s Baseball Barn. He also travels and shares his story of what God has done in his life, in and out of baseball.