The control of the valley degree of freedom lies at the core of interest in monolayer transition metal dichalcogenides, where specific valley-spin excitation can be created using circularly polarized light. Measurement and manipulation of the valley index has also been achieved, but mainly with purely optical methods. Here, in monolayer MoS_{2}, we identify a response to the valley polarization of excitons in the longitudinal electrical transport when the valley degeneracy is broken by an out-of-plane magnetic field B_{z}. The spin information is also simultaneously determined with spin-sensitive contacts. In the presence of B_{z}, a significant modulation of the photocurrent is observed as a function of the circular polarization state of the excitation. We attribute this effect to unbalanced transport of valley-polarized trions induced by the opposite Zeeman shifts of two (K and K^{'}) valleys. Our interpretation is supported by the contrasting behavior in bilayer MoS_{2}, as well as the observed doping and spatial dependence of the valley photocurrent.