The crystallographic and magnetic properties of LaNi5−xMnx with x=1, 1.5, and 2, have been investigated using x-ray and neutron diffraction, vibrating sample magnetometer and superconducting quantum interference device measurements. All the samples crystallize in the hexagonal CaCu5-type (P6/mmm) structure. Manganese atoms preferentially occupy the 3g site in the LaNi5 structure. The bulk magnetization of the LaNi5−xMnx powder samples decreases rapidly as nickel is replaced by manganese. A ferrimagnetic model is applied to describe the magnetic structure of the LaNi5−xMnx samples for x=1.5 and 2. A ferromagnetic model gives the best fit of the neutron diffraction data for the LaNi4Mn sample.