The No. 10 Target Chip Ganassi Racing driver crossed the finish line 0.1430 seconds ahead of rookie Hideki Mutoh.
Wheldon won for the 15th time in his IndyCar career by driving the last 90 laps without making a pit stop and 37 laps under a caution flag.
His winning strategy started when the caution flag came out with 62 laps left, with most of the leaders making pit stops except for Wheldon, Mutoh and Danica Patrick.
"Now that's a birthday worth remembering," Wheldon said.
Both Wheldon and Dixon gave their winnings to the Iowa flood relief effort.
Dixon leads Helio Castroneves by 48 points in the IRL point standings and Wheldon, with his win, holds third place.