Alonso, 37, retired from F1 at the end of the season but won a rain-affected race alongside Renger van der Zande, Kamui Kobayashi and Jordan Taylor in the Wayne Taylor Racing Cadillac.
Alonso is the third F1 champion to win after Phil Hill and Mario Andretti.
A result was finally declared after 23 hours and 10 minutes, with the Cadillac of Felipe Nasr in second and the Penske of Ricky Taylor in third.
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