Yokohama travelled to Kawasaki needing a win to maintain their slender lead atop the division and they had to wait no more than eight minutes for Teruhito Nakagawa to break the deadlock.
Nakagawa's effort moved to calm Postecoglou's side at the Todoroki Athletics Stadium and their afternoon only got better in the second half; Erik Lima striking twice in 20 minutes to stun the defending champions.
Though Leandro Damiao's 74th-minute goal brought Frontale back into the contest, it proved mere consolation as Keita Endo restored Yokohama's three-goal advantage with mere minutes remaining.
While Postecoglou will take plenty of pride in the performance, other results around the league have since eased the pressure surrounding next week's final hurdle at home to second-placed FC Tokyo.
Tokyo's 1-1 draw at home to Urawa Red Diamonds means Yokohama now hold a three-point lead over their rivals, with third-placed Kashima now out of the title race after falling 3-1 to Vissel Kobe.
As a result, Postecoglou's side will need to avoid a four-goal defeat if they are to clinch a historic league title next Saturday.
Success would mark the first time an Australian manager has claimed Japanese football's biggest prize.
