Making his tournament debut and playing just the sixth major of his career, Young carded eight birdies in a flawless 64.
Mickelson, one of the players to have joined the Saudi-backed breakaway competing at St Andrews, told a reporter to “let it go”.
The LIV Golf rebel was booed as he began his challenge at St Andrews.
Poulter milked the applause after he holed from 160 feet.
The 150th Open Championship did not start well for Englishmen.
The 150th Open Championship gets under way on Thursday.
It is 23 years since the Scot lifted the Claret Jug at Carnoustie.
The Ryder Cup star was told his ball was stuck up a tree at the Genesis Scottish Open.
The fairways are running hard and fast on the Old Course.
R&A chief executive Martin Slumbers wants the event to remain “open to anybody”.
