National Association of Base Ball Players: Hudson River of Newburgh, NY (1864–1865); Irvington of Irvington, NJ (1866–1867); Buckeye of Cincinnati (1868); (1869–1870)
League player: Washington Olympics (1871); Boston Red Stockings/Red Caps (1872–1878); Cincinnati Reds (1880)

Andrew Jackson Leonard (June 1, 1846 – August 21, 1903) played left field for the original , the first fully professional baseball team. He was one of five men to play regularly for both the Cincinnati and the Boston Red Stockings, the latter winning six championships during his seven seasons. He played several infield positions on lesser teams in his early twenties but left field was his regular professional position.

Born 1846 in County Cavan and raised in Newark, New Jersey, Leonard is commonly recognized as the first native of Ireland to play in the major leagues. He was one of four who played during the first National Association season, so he owes the distinction partly to fortunate scheduling in the spring of 1871, partly to our counting the NA as a major league. (But it seems likely that Leonard and Fergy Malone both played in the first National League game, 22 April 1876.)

Leonard played five seasons in the amateur era of the National Association of Base Ball Players (NABBP), beginning in 1864 with the Hudson River club of Newburgh, New York, not far North of the metropolis. Early in the 1866 season he moved to the Irvington club of Irvington, New Jersey, several miles inland from Newark and from the Elysian Fields, Hoboken, where many New York city teams played home matches. (At least two teammates, Hugh and Matt Campbell, were natives of Ireland.) At that time, in his early twenties, the right-handed Leonard played mainly in the infield.

In 1868 "Andy" and teammate Charlie Sweasy moved to Cincinnati, Ohio, and joined the Buckeyes, the chief local rival of the ; the move suggests that he was somehow compensated by club members if not by the club.

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