Brazilian winger Galeno put the hosts in front in the 35th minute with a curling strike into the top corner before a Franck Kessie header doubled his side's advantage over the J-League side three minutes before the interval.
The victory makes Al Ahli the third club from Saudi Arabia to win the continental title after previous successes by Al Hilal and Al Ittihad.
Al-Ahli went into the final in front of their own fans as heavy favourites with a lineup featuring an expensively acquired array of foreign talent that includes UEFA Champions League winners Riyad Mahrez and Roberto Firmino.
Twice inside the opening six minutes Louis Yamaguchi denied the home side, the Kawasaki goalkeeper blocking Ivan Toney's viciously struck right-foot effort before making a point-blank block at the resulting corner from Ziyad Al-Johani.
The Japanese outfit almost silenced the crowd in the 11th minute with a moment of individual skill from Marcinho, the Brazilian gliding beyond Ali Majrashi to bear down on goal only to shoot across the area and past the far post.
Toney fired wide and Roger Ibanez was just off target with a curling effort from the edge of the area as Al-Ahli increased the pressure, but a disciplined Kawasaki held firm.
However, the opener eventually came with 10 minutes remaining of the first half when Kawasaki gave away possession close to their own goal.
Firmino fed Galeno in space 25 metres out and, as midfielder So Kawahara made a late, vain attempt to close the Brazilian down, the former Porto man bent a perfectly-measured right-foot strike into the top corner.
Al-Ahli doubled their lead soon after, the Saudi side capitalising on their opponents being temporarily down to 10 men with left back Sota Miura off the field injured.
Firmino was heavily involved again, bursting through on the right side of the area to clip a cross into the six-yard box that the quickly arriving Ivorian Kessie rose to head home.
Substitute Sai van Wermeskerken nodded wide as Kawasaki sought a way back into the game and substitute Tatsuya Ito, the hero of Kawasaki's semi-final win over Al-Nassr, shot just wide with 15 minutes remaining.
Ito flashed another strike across Edouard Mendy's goal and Ibanez denied Shin Yamada a late opportunity as Al-Ahli celebrated victory in their third appearance in the final, after runners-up finishes in 1986 and 2012.