McLean's eight-under total matched that of Walker Cup youngster and playing partner Stiggy Hodgson (Sunningdale) as they finished five shots behind the winner, Matthew Southgate (Thorpe Hall) of Essex.
"That's my best in five attempts at this tournament," said McLean, Edward Trophy winner earlier in the season. "I was determined to do better this weekend than I've done in the past so I came down to St Andrews on the Wednesday and got in some good practice rounds.
"I was hitting the ball pretty straight and, looking back, I could have finished second on my own but for a double bogey 6 with a lost ball at the second in the final round. But I can't complain really. That spurred me on to birdie the third, fourth, fifth, 10th, 12th, 13th, 16th and 18th.
"I knew it was between me and Stiggy for second place and when he holed from about 20ft for a birdie at the last I knew I had to follow him in from seven feet, also for a birdie, which I did. So that was a nice way to end. I'm going home to Peterhead with some good memories."
Southgate, 21, has yet to gain a full England cap, his biggest win prior to this being in his home county, at the Essex open amateur championship.
"I couldn't have picked a better place for my biggest win," said Southgate who shared the lead at halfway with Denmark's Daniel Lokke and Ben Westgate (Trevose) on six-under-par 138.
Hodgson had taken the lead after a third-round 67 for 207, a one-shot lead over Southgate who forged clear on his own to win by five shots with a closing 67 (34-33) for a 13-under-par total of 275.
Second Scot was Michael Stewart from Troon Welbeck, back for the summer from East Tennessee State University, who finished joint seventh on 282 with Sunday rounds of 72 and 69.
Stirling University student James White (Lundin) shared 15th place on 284, closing with 70 and 71, while Matthew Clark (Kilmacolm) finished joint 24th on 286 with rounds yesterday of 67 and 75.