A facile and efficient method is presented for the synthesis of 6‐alkylated phenanthridine, possessing hindered quaternary carbon centers, by Zn‐mediated reductive trapping of tertiary alkyl radicals with both electron‐rich and electron‐deficient aryl isocyanides using nickel as a promoter. Bench‐stable and operation‐friendly tertiary alkyl oxalates derived from abundant tertiary alkyl alcohols were first used as radical precursors for the alkylation of isocyanobiphenyl species. This reaction displays excellent functional group tolerance and broad substrate scope, allowing access to desired products in good to excellent yields. The efficient radical cascade cyclization protocol for the reductive cleavage of C−O bonds followed by phenanthridine formation is reported. The method involves generating tertiary alkyl radical intermediates that are invoked by single‐electron reduction by Zn, wherein MgCl2 serves as the indispensable additive and nickel as the promoter. The reaction displays excellent functional group tolerance and broad substrate scope.