Field assisted sintering (FAST) has demonstrated great potential in reducing temperature constraints imposed on ceramic materials during sintering. While there have been many phenomenological observations of FAST processes, mechanistic understanding of the effect of electric field has been understudied and remains open. In particular, quantifying cation diffusion at different temperatures and polarizations both in bulk and at grain boundary is important for resolving the densification process under electric field. To that end, we studied the effect of defect chemistry on the field assisted sintering of TiO2 by both sintering experiments and first-principles based modeling. Sintering experiments under electrical field were conducted inside a dilatometer to control the shrinkage of the material in real time. To separate out the Joule heating effect, we used controlled doping concentration and oxygen partial pressure to controllably vary the electronic conductivity of TiO2, thereby controlling the extent of the Joule heating contribution. By varying the main cationic defects between Ti vacancies and interstitials, as well as their concentrations, we studied their effect on the FAST process. In parallel, the equilibrium defect concentrations under relevant conditions were predicted utilizing a first-principles based computational framework. By combining equilibrium defect concentration with defect diffusivities obtained from force field molecular dynamics, we constructed Ti diffusion profiles as a function of temperature, oxygen partial pressure and electric polarization. The predicted Ti self-diffusion coefficient agrees reasonably well with experimental measurements and in both cases Ti interstitial contributes dominantly. Moving from bulk to grain boundary, we found a very weak space-charge effect and a three-order-of-magnitude increase in the diffusion coefficient due to decreased Ti migration barrier. This result implies that Ti diffusion is greatly accelerated at the grain boundary. Identifying these factors advances our understanding of the individual effects of Joule heating, bulk cation diffusion, grain boundary mobility and electric polarization on flash behavior, and thereby, guide better control of ceramic materials manufacturing by FAST technology.