A silver-catalyzed tandem carbon-carbon triple bond hydroazidation, radical addition, and cyclization of biphenyl acetylene is described under mild conditions, leading to the formation of 6-methyl sulfonylated phenanthridines in good yields. In this novel cascade reaction, most of the atoms are incorporated into the product without cleavage of the C≡C bond. Mechanistic studies suggest the reaction should proceed through an iminyl radical reactive intermediate.