Mackinnon confirmed the news on Twitter, also stating that pre-production begins this Monday (16th December). Mackinnon has directed four episodes - 2008 two-parter The Sontaran Stratagem and The Poison Sky, 2012's penultimate Pond adventure, The Power of Three, and most recently, this year's revival of the Ice Warriors in Cold War.
Mackinnon will be directing two episodes of Series 8, joining Ben Wheatley (Episodes 1 & 2) and Paul Murphy (Episodes 3 & 6) - presumably, Mackinnon's episodes will be Episodes 4 and 5, and could be the first to be filmed of the new series.
Series 8, starring Peter Capaldi as the Doctor, will be broadcast in Autumn 2014, and the series is expected to contain 12 episodes.
[Source: Doctor Who TV]