A cantilever magnetometer is described that measures the magnetic moment of ferromagnetic films with submonolayer sensitivity. The magnetometer is incorporated into an ultrahigh vacuum chamber for sample preparation and in situ magnetometry. Standard size single crystals of 5 mm diameter and 2 mm thickness can be used, which are mounted on thin sheet metal. This composite sampleholder works as a cantilever when the bending is induced by the torque exerted by an external magnetic field on a monolayer ferromagnetic film deposited onto the single crystal substrate. We demonstrate the submonolayer sensitivity on Fe monolayers on Cu(100) in the thickness range from 2 to 68 monolayers. The sample holder is designed for internal calibration by passing a current through it and exploiting the well-known current induced magnetic moment.