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.