While Mn-catalyzed (de)hydrogenation of carbonyl derivatives have been well established, the reactivity of Mn hydrides with olefins remains very rare. Herein we report the first Mn(I) pincer complex that effectively promotes site-controlled transposition of olefins. This new reactivity is shown to emerge once the N-H functionality within Mn/NH bifunctional complex is suppressed by alkylation. While detrimental for carbonyl (de)hydrogenation, such a masking of the cooperative NH functionality allows for the highly efficient conversion of a wide range of allylarenes to higher-value 1-propenybenzenes in near-quantitative yield with excellent stereoselectivities. The reactivity towards a single positional isomerization was also retained for longer-chain alkenes resulting in the highly regioselective formation of 2-alkenes, which are less thermodynamically stable compared to other possible isomerization products. The detailed mechanistic analysis of reaction between activated Mn catalyst and olefins points to catalysis operating via a metal alkyl mechanism - one of the three conventional transposition mechanisms previously unknown in Mn complexes.