Incorporating the molecular organic Lewis acid tris(pentafluorophenyl)borane [B(C6F5)3] into organic semiconductors has shown remarkable promise in recent years for controlling the operating characteristics and performance of various opto/electronic devices, including, light-emitting diodes, solar cells, and organic thin-film transistors (OTFTs). Despite the demonstrated potential, however, to date most of the work has been limited to B(C6F5)3 with the latter serving as the prototypical air-stable molecular Lewis acid system. Herein, the use of bis(pentafluorophenyl)zinc [Zn(C6F5)2] is reported as an alternative Lewis acid additive in high-hole-mobility OTFTs based on small-molecule:polymer blends comprising 2,7-dioctyl[1]benzothieno [3,2-b][1]benzothiophene and indacenodithiophene–benzothiadiazole. Systematic analysis of the materials and device characteristics supports the hypothesis that Zn(C6F5)2 acts simultaneously as a p-dopant and a microstructure modifier. It is proposed that it is the combination of these synergistic effects that leads to OTFTs with a maximum hole mobility value of 21.5 cm2 V−1 s−1. The work not only highlights Zn(C6F5)2 as a promising new additive for next-generation optoelectronic devices, but also opens up new avenues in the search for high-mobility organic semiconductors.