The minor setback did little to disrupt the 50-year-old's preparation as he became the oldest Major winner ever.
Mickelson Cracked His 2-Iron Minutes Before PGA Championship Final Round
As Phil Mickelson entered the final round of the PGA Championship with a one shot lead, his 2-iron cracked during his warm-up on Sunday.
After handing the 2-iron back to his brother and caddie Tim Mickelson, Tim noticed the crack immediately.
Related: Who Is Phil Mickelson's Caddie?
With just minutes left until Mickelson teed off for the final round, Tim hurried back to the clubhouse to retrieve a spare 4-wood Mickelson had brought along with him.
Despite not having practised with the club since Tuesday, the 50-year-old decided to put the club into the bag and used iton the third and fourth holes.
The oldest Major champion ever suggested that the club worked effectively, even though he only achieved a bogey and par on those holes.
"As I was teeing off today my 2-iron face cracked," Mickelson said during his post-tournament interview.
He then joked about how his power caused the club to crack.
"I mean, you can't swing it as hard as I hit it and not expect them to crack - I'm kidding."
However, there is an element of truth in Mickelson's comments about his power.
He hit the longest shot on the 16th hole all week, driving the ball an incredible 366 yards.
Related: Phil Mickelson What's in the bag?
Mickelson described how Tim first noticed the cracked club after he handed it back to him.
"Tim noticed when I put it back that it had cracked across the face," he said.
"Fortunately I had a 4-wood that's a very comparable club to that 1-iron distance-wise and I was able to use that club effectively.
"I used it off 3 tee, 4 tee, and there was a few times that I hit it and I hit that club very well.
"It's just one of those things that happens and you just have to be prepared for it, which is why I bring backup clubs out here."
All players will use a full 14-club set, though the six-time Major winner is known for travelling with up to 25 clubs in case of changing weather conditions or emergencies.