High-resolution neutron powder diffraction and transverse field mu SR measurements have been used to examine the nature of the magnetic and structural transition at 100 K in YMn₂. The neutron measurements indicate a first-order hysteretic transition occurring at 92.5 K on cooling and 108 K on warming. The low-temperature phase is tetragonal (c/a=0.995) and antiferromagnetic with localized Mn moments of 2.8 mu _B ordering in a long-wavelength helix characterized by an antiferromagnetic translation vector ( tau , tau ',1) where tau =0.018 and tau '=0.003. The high-temperature phase is cubic and Pauli paramagnetic. The collapse of the Mn moment at the transition is accompanied by a 5% decrease in the cell volume. In the paramagnetic state of YMn₂ measurement of the muon transverse relaxation rate reveals an underlying second-order transition which the authors associate with the critical slowing of longitudinal spin fluctuations as the transition temperature is approached.