The magnetic and magnetocaloric properties of the new HfFe6Ge6-type (P6/mmm) MgMn6Ge6 compound have been studied by magnetic measurements and powder neutron diffraction experiments. MgMn6Ge6 magnetically orders at TC∼345K in a ferromagnetic easy-plane arrangement. At TAF∼235K, it undergoes a first-order transition to a helical structure, characterized by a temperature dependent propagation vector k=<0,0,qz> (qz∼0.116 reciprocal lattice units at 1.4 K), without structural modification. The Mn atoms carry a magnetic moment of about 2μB. A metamagnetic transition with a low critical field (Hcr≪1T) is observed below TAF. At low temperature, both magnetic and magnetocaloric data strongly suggest that some kinds of intermediate noncollinear ferromagnetic arrangements are stabilized above the critical field while the pure ferromagnetic state is obtained for significantly higher magnetic fields (H≫5T). The magnitude of the magnetic entropy change at TC (-ΔSM∼20.5mJ cm-3 K-1 for ΔH=5T) is about 25% that of Gd metal. The magnetocaloric effect at- the order-order transition is of opposite sign and of lower magnitude.