Modulation of the electron transport properties of metallopolymers based on nickel-salen complexes N,N′-bis(salicylidene)ethylenediaminonickel(II) and N,N′-bis(3-methoxysalicylidene)ethylenediaminonickel(II) upon electrochemical p-doping was studied by in situ conductance measurements in an organic electrochemical transistor configuration. Both polymers showed bell-shaped conductance profiles upon positive voltage bias; the results were interpreted using the mixed-valence conductivity model. The electrochemical doping of the methoxy-substituted polymer film resulted in a change in its electrical conductance by four orders of magnitude, which indicates its potential suitability as a channel material for electrochemical transistors. Electrochemical quartz crystal microbalance studies confirmed mixed ionic-electronic transport in this polymer film and revealed different regimes of ion and solvent transfer in the polymer at different doping levels.