The aim of this paper is to elucidate the origin of the transition from twinning to slip dominated flow in pure Mg. With that purpose, two polycrystals with average grain sizes of 19 and 5 μm were prepared by rolling and annealing and they were tested in compression along the rolling direction at strain rates ranging from 10−3 to 10−5 s−1 and at temperatures comprised between 50 and 250 °C. Twinning was evaluated by conventional electron backscatter diffraction (EBSD) and the activity of different slip systems was measured by an exhaustive EBSD-assisted slip trace analysis. A transition from twinning to basal slip, localized along deformation bands, was found to take place with decreasing grain size, with decreasing strain rate and with increasing temperature. The emergence of basal slip as the dominant deformation mechanism is promoted in all three cases by increasing levels of connectivity between favorably oriented grains, which facilitate slip transfer across grain boundaries. Such connectivity is related to the fraction of grain boundaries (GBs) with misorientations smaller than a threshold value (fθ < θth) as well as to their local arrangement. Since processing for grain refinement results in larger fθ < θth values and both decreasing strain rate and increasing temperature increase θth, in all cases twinning is eventually replaced by basal slip.