The 24-year-old is the eleventh American to lift the Stonehaven Trophy.
Leishman was tied third with Jasper Stubbs on 14-under, and took the third available qualifying spot courtesy of his higher position in the Official Golf World Rankings.
The 41-year-old hasn’t competed in the Championship since The 150th Open, at St Andrews in 2022, but he did finish T2 in 2015.
The ISPS Handa Australian Open was the first event of The Open Qualifying Series, which offers golfers around the world opportunities to book their place in golf’s original Championship.