The mechanism of [{RuCl2(p‐cymene)}2]‐catalyzed oxidative annulations of isoquinolones with alkynes was investigated in detail. The first step is an acetate‐assisted CH bond activation process to form cyclometalated compounds. Subsequent mono‐alkyne insertion of the RuC bonds of the cyclometalated compounds then takes place. Finally, oxidative coupling of the CN bond of the insertion compounds occurs to afford Ru0 sandwich complexes that undergo oxidation to regenerate the catalytically active RuII complex with the copper oxidant and release the desired dibenzo[a,g]quinolizin‐8‐one derivatives. All of the relevant intermediates were fully characterized and determined by single crystal X‐ray diffraction analysis. The [{RuCl2(p‐cymene)}2]‐catalyzed CH bond functionalization of isoquinolones with alkynes to synthesize dibenzo[a,g]quinolizin‐8‐one derivatives through CH/NH activation was also demonstrated.