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We report on the magnetic and magneto-optical properties of the intermetallic alloys Mn5(Ge1-xMx)3, where M=Sn, Pb. Generally, these alloys were ordered ferromagnetically at T≥300 K for x<0.3. The structure of Mn5(Ge1-xSnx)3 remains in the hexagonal Mn5Si3 (hP16) phase up to x=0.25 and then changes to Ni2In (hP6) as x approaches 1. The magnetization and Curie temperature of these alloys is largest for small x and reach a minimum value near x=0.5. The room temperature Polar Kerr rotation, however, increases from –0.06° at x=0 to maximum value of –0.12° at x=0.1 before decreasing toward zero for x≥0.3. The Mn5 (Ge1-xPbx)3 samples show a Mn5Si3 structure with ferromagnetic behavior for x≤0.3, and varying structure with rapidly decreasing magnetization above x=0.3. The magnetic behavior observed was complex for x≥0.3. The Kerr rotation of the Pb-doped alloys in the ferromagnetic region monotonically decreases from –0.06° to zero at x≃0.3.