By coupling an improved speckle patterning method enabling high resolution digital image correlation (HRDIC) at nanoscale strain resolution (146 nm) with a scanning electron microscope allowing autonomous experimental control and image acquisition during in situ tensile straining, we have mapped the plastic deformation in AZ31 Mg alloy at the grain scale to significant plastic strains for the first time. Complemented by electron backscattered diffraction to reveal the underlying grain orientations this has delineated the activation of intragranular extensional twinning, slip, and intergranular deformation mechanisms. Using a new grain boundary (GB) strain mapping algorithm, displacements tangential to the GBs as large as 140 nm at 2% plastic strain have been recorded. Critically, this occurs not so much by sliding at the boundary but by slip over a mantle region extending 450 nm from the GBs. Within the grain interiors, dislocation activity is much less, taking place mainly by basal slip. Reverse dislocation slip and detwinning were found to occur on unloading emphasising the need for in situ measurements. The proposed methodologies (gold speckle pattern remodelling, automated in situ HRDIC tensile testing and GBS activity analysis) have the potential to characterise the real-time deformation behaviour of a wide range of engineering alloys at the grain scale at room and elevated temperatures.