ST. ANDREWS, Scotland — Don’t let Cam Smith’s look and demeanor fool you.

The pencil mustache. The mullet. The carefree attitude that suggests he doesn’t really give a damn about much of anything other than chilling on his boat down in Jacksonville Beach, Fla., where he lives.

Smith, the 28-year-old from Queensland, Australia, is a fearless assassin with golf clubs in his hands. And when he gets hot on the golf course, particularly with his putter, he cannot be stopped.

Rory McIlroy, who shared a four-shot 54-hole lead with Viktor Hovland, found that out on Sunday in the final round of the 150th British Open at St. Andrews.

So did Smith’s playing partner, Westchester’s Cameron Young, who shot a 7-under 65, including a 5-under 31 on the back nine.

McIlroy’s four-shot lead and his bogey-free, 2-under final-round 70 wasn’t enough and neither was Young’s final-round 65.

McIlroy heard Smith’s footsteps playing in the pairing ahead of him, and Young witnessed it live and in frightening living color as Smith delivered an overwhelming clutch performance with an 8-under 64 to capture the Claret Jug, his first major championship triumph and one that was seized with style.

Smith edged out Young, who eagled the 72nd hole to get to 19-under, with a game-winning birdie on 18 to finish 20-under par. McIlroy, who’s been waiting eight years since his last major championship victory, was third at 18-under. Hovland struggled in the scalding spotlight of playing in the final group, shooting 74.

Smith played a back nine for the ages, carding birdies on five consecutive holes — Nos. 9 through 14 — to seize the lead from McIlroy.

“It was pretty tense,’’ Smith said. “I think maybe after my second or third birdie there on the back nine, I was starting to think that I could really win this thing. I was three back with nine holes to go and I really needed to make something happen.

“From there, I was starting to get different emotions and really had to keep an eye on what I was thinking and just different shots into greens.’’

Smith, it should be noted, won the Players Championship in March on TPC Sawgrass, a target golf course that cannot be any more different than the one Smith plotted his way around and conquered on Sunday.

This is the best illustration of how good this guy is. He’s a big-game hunter who can win on any type of golf course in any conditions.

“Some people have it,’’ fellow Aussie Adam Scott said. “He’s got it, he wants it and he’s not afraid of it. He’s shown that he’s and for it. He’s a good mate and I’m just thrilled for him.

“This might be as special as it can get — winning an Open Championship at St. Andrews, and he’s done it kind of in the finest style possible.’’

Smith also shot 64 on Friday and made a PGA Tour record 255 feet in putts. But his putting left him on Saturday as he shot a 1-over 73 and lost the lead, making only 50 feet of putts in that third round.

The putter returned on Sunday, just in time.

“For me luckily, putting comes quite naturally,’’ Smith said. “I practice with a mirror for probably 20 minutes a day, and to be honest, that’s about it.’’

Smith said the long birdie putt he made on No. 11 to get to 16-under spurred him on.

“When that one dropped, I could see the hole getting a lot bigger on that back nine for sure,’’ he said.

“I had a front-row seat to I’m sure one of the better rounds that’s been played this year,’’ Young said. “Today kind of just is more proof that he is that good and he is one of the very, very best players in the world.’’

Smith might have won the tournament on No. 17, the famous Road Hole, when his approach shot fell short of the green and awkwardly near the Road Hole Bunker. A chip from the tight lie onto the shallow green was risky. He opted to putt, even though he knew the closest he could get it to the hole was 10 or 12 feet for par.

His par save there was crucial, keeping him a shot ahead of McIlroy.

“Coming down on 14, I knew that at that point Cam had birdied to go to 19 and I was at 18, so I knew that I needed to respond,’’ McIlroy said. “I just couldn’t find the shots or the putts to do that.’’

With Smith carding birdie on 18, McIlroy was two shots back and needed eagle. He failed to drive the par-4 18th and needed to hole a chip to force a playoff, which he didn’t.

“Disappointed obviously,’’ McIlroy said. “I felt like I didn’t do much wrong today, but I didn’t do much right either.’’

No one did as much right as Smith, a worthy winner.

“He doesn’t have that wow factor when you look at him,’’ Hovland said. “It’s just unbelievable how he’s able to just get the ball in the hole. He’ll hit a bad shot and just doesn’t seem to bother him because he knows that he’s going to hit a great next shot. That’s what golf is all about. He’s a worthy champion for sure.’’