The 52-year-old Victorian recovered from a slow start to his final round with a closing nine of 33 to finish four shots behind Bernhard Langer who won the event for the fourth time with a final round of 66.
McKenzie is playing his second full season on the Champions Tour but until this week had recorded only one top ten in 12 starts in 2019 and this performance will go some way towards securing his status for 2020.
PGA Tour Champions rookie, Stephen Leaney, was the next best Australian in a tie for 18th.
For Langer it was his 11th senior major victory and his 41st title since turning 50.
"It's always special to walk down 18 in a major, but especially over here,” said the two-time Masters Champion. “The people are so knowledgeable about golf, and it's tremendous how they even came out in these conditions and cheered us on in the rain.
“It feels very special, whether it's The Open or The Senior Open, to walk down 18 is always a wonderful feeling.
“Now having won 11 senior majors, it means a great deal. Nobody has won more than nine if I'm correct, not even the great Jack Nicklaus or Gary Player or Tom Watson who retired from this championship today.
“There's a lot of great players who have played this game and have played majors, and I've been blessed to have won more than any of them.”
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