Due to their light weight, magnesium (Mg) and its alloys have great potential for reducing vehicular mass and energy consumption. However, the use of Mg alloys is currently restricted to low-temperature automotive components application. This work strives to gain a better understanding of the effect of yttrium (Y) (up to 3at%) on creep behavior of columnar nanocrystalline Mg with a grain size of 5nm and 10nm. Using molecular dynamics (MD) simulations, nanocrystalline Mg with various local concentrations of Y was subjected to a constant-stress loading, ranging from 0-500MPa, at different initial temperatures, ranging from 473-723K. Our simulations reveal that the secondary stage creep rate (ε) decreases by 71% with the addition of only 1at% Y at 500MPa and 623K. With the addition of alloying elements such as Y, the creep rate in the secondary region decreases and the creep deformation mechanism is changed from the void nucleation, growth, and coalescence to GB rotation/sliding. The analyses of the diffusion coefficient and energy barrier reveal a stronger contribution to the overall deformation by the grain boundary diffusion at the low-temperature (423K) and by the lattice diffusion at the higher-temperature (723K).