Bernard McMahon or M'Mahon (Ireland ca 1775 — Philadelphia, 18 September 1816) was an Irish-American horticulturist settled in Philadelphia, who served as one of the stewards of the plant collections from the Lewis and Clark expedition and was the author of The American Gardener's Calendar: Adapted to the Climates and Seasons of the United States (1806 and following years). He circulated the first extensive gardener's seed list in the United States, which he attached as an appendix to his Calendar. Jefferson received from McMahon seeds for Monticello.