Molecular dynamics computer simulations have been carried out of a chemically realistic many-chain nonentangled model of glassy atactic polystyrene under the influence of uniaxial mechanical deformation. Both the initial elastic and the postyield (up to 100% of the deformation) behavior have been simulated. The Poisson ratio, the Young modulus, and the temperature dependence of the yield peak are well reproduced. The simulated strain-hardening modulus is in quantitative agreement with existing experiments. The deformationally induced anisotropy in the global and local segmental orientation is accompanied by an anisotropy of the local translational mobility: the mean-square translational displacement of the individual segments in the direction of the deformation is drastically increased just beyond the yield point as compared to the isotropic sample. The mechanical deformation of a quenched sample leads to an almost complete erasure of the aging history.