New electrochemical ammonia (NH3) synthesis technologies are of interest as a complementary route to the Haber-Bosch (HB) process for distributed fertilizer generation, and towards exploiting ammonia as a zero-carbon fuel produced via renewably-sourced electricity.1–4 Apropos of these goals is a surge of fundamental research targeting heterogeneous materials5–7 as electrocatalysts for the nitrogen reduction reaction (N2RR). These systems generally suffer from poor stability and NH3 selectivity; competitive hydrogen evolution reaction (HER) outcompetes N2RR.8 Molecular catalyst systems can be exquisitely tuned and offer an alternative strategy,9 but progress has thus far been thwarted by the same selectivity issue; HER dominates. Herein we describe a tandem catalysis strategy that offers a solution to this puzzle. A molecular complex that can mediate an N2 reduction cycle is partnered with a co-catalyst that interfaces the electrode and an acid to mediate concerted proton-electron transfer (CPET) steps, facilitating N−H bond formation at a favorable applied potential and overall thermodynamic efficiency. Without CPET, certain intermediates of the N2RR cycle would be unreactive via independent electron transfer (ET) or proton transfer (PT) steps, thereby shunting the system. Promisingly, complexes featuring several metals (W, Mo, Os, Fe) achieve N2RR electrocatalysis at the same applied potential in the presence of the CPET mediator, pointing to the generality of this tandem approach.