McKinney birdied the second playoff hole at Dundonald Links in Ayrshire, Scotland, to lock in his spot for what will be the 21-year-old's major debut.
The 2022 Australian Amateur champion was three under through 36 holes to finish in a tie for third before surviving the playoff.
He'll join a strong Australian assault at Royal Liverpool headed by defending Open champ Cameron Smith.
Nineteen players from a starting field of 288 secured their Open spots at qualifying events in Dundonald, Royal Cinque Ports, Royal Porthcawl and West Lancashire.
"To play in golf's oldest championship as your first major and having grown up watching The Open - I have been to a few watching all the great names coming through and having been to St Andrews a bunch - is pretty special," said McKinney, who was born in Scotland but moved to Australia at the age of 12.
"It's something I have always dreamed of. You don't really know when it's going to happen.
"You have to envision it but it's a pretty surreal feeling to be sure."
The other Australians in the field are Smith, Min Woo Lee, Adam Scott, Haydn Barron, Harrison Crowe, David Micheluzzi, Jason Day, Lucas Herbert and Travis Smyth.
There will be more Open spots up for grabs at next week's Scottish Open on the DP World Tour.
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