Magnetic properties of amorphous (Ni1-xMnx) 75P16B6Al3 alloys have been probed using transverse magnetoresistance (in fields up to 9 T), and susceptibility measurements (in low fields using a SQUID magnetometer, and at high fields of up to 1 T using a Faraday balance), in the temperature range 4-300 K. Low field susceptibility data show a spin-glass type of behavior for alloys with x>0.1. At low concentrations of Mn the spin freezing temperature Tf is found to increase linearly with x. A maximum value of Tf=61 K is observed for the alloy with x=0.6. Analyses of the high field susceptibility data indicates that in this amorphous matrix Ni does not carry a moment. The transverse magnetoresistance is found to be negative at all temperatures and applied fields, and for T>2Tf the magnetoresistance is found to have a quadratic field dependence.