Monolayers of semiconducting transitional metal dichalcogenides (TMDC) are emerging as strong candidate materials for next generation electronic and optoelectronic devices, with applications in field-effect transistors, valleytronics, and photovoltaics. Prior studies have demonstrated strong light-matter interactions in these materials, suggesting optical control of material properties as a promising route for their functionalization. However, the electronic and structural dynamics in response to electronic excitation have not yet been fully elucidated. In this work, we use non-adiabatic quantum molecular dynamics simulations based on time-dependent density functional theory to study lattice dynamics of a model TMDC monolayer of MoSe2 after electronic excitation. The simulation results show rapid, sub-picosecond lattice response, as well as finite-size effects. Understanding the sub-picosecond atomic dynamics is important for the realization of optical control of the material properties of monolayer TMDCs, which is a hopeful, straightforward tactic for functionalizing these materials.