Exxon Mobil Corp. was stripped Tuesday of the perfect triple-A credit rating it has held for more than six decades by Standard & Poor’s Ratings Services, a sign that the oil bust is hurting even the strongest players in the energy industry.

The world’s largest publicly traded oil company, Exxon, was just one of three American companies—Microsoft Corp. and Johnson & Johnson are the others—that had the triple-A rating. S&P said it first gave the company the triple-A mark in 1949. A representative for Exxon said...