The John Mason House is a historic First Period house in Lexington, Massachusetts. It is a 2+1⁄2-story wood-frame structure, its main block three bays wide, with a side-gable roof, clapboard siding, and two chimneys set behind the roof ridge. A leanto section to the rear gives it a saltbox profile, and an ell extends to the right. The oldest portion of the house, a three-bay portion of its main block, was built circa 1715. The building has been repeatedly extended and altered over the years. John Mason, its builder, was a prominent local citizen.