Nobody has ever won a major championship at the age Mickelson is at now, but then again, not too many players have had the confidence to play two holes essentially blindfolded - as he wanted to do to finish this off Sunday night instead of Monday morning.
With light fading, Mickelson was keen to finish the round, saying: "I can see fine" but England's Casey opted not to putt on the 16th.
Hale Irwin, who won the 1990 edition of the US Open at 45 years and 15 days old, is the oldest US Open victor to date so the stats are really against Phil Mickelson, who turns 49 on the Sunday of this year's edition.
Even a dawn wakeup call didn't bother Phil Mickelson as he birdied the final hole Monday to claim the title at AT&T Pebble Beach Pro-Am. "Sometimes I get in my little bubble and don't see the big picture".
The weekend's PGA Tour event is still now ongoing and scheduled for a Monday finish due to adverse weather conditions over the weekend causing delays.
"So at the end of previous year, even though I played poorly, I had something happen where it seemed like overnight - it had really been in the works - where my driver speed shot up 5, 6 miles an hour, which rarely ever happens to anybody, let alone somebody in their late 40s", he said.
Scott Stallings, who took the clubhouse lead on Sunday by getting to 15 under, was third, with Jason Day and Kim Si-woo tied for fourth at 13 under.
Mickelson sits at -18 for the tournament, with three holes to play.
It also would be his first victory on American soil since the Phoenix Open six years ago.
He has gone 28 years since his first PGA Tour victory and his 44th, and he joined Tiger Woods as the only players to surpass $90 million in career earnings.
He will return to Pebble Beach in June for the US Open, where he made his pro debut in 1992. "I'll deal with that in six months".
After his 9-iron into a foot at No. 9 to get within one shot, Mickelson holed a 12-foot birdie on the 10th with a drive that hugged the right side of the fairway and likely would have bounced into the ocean if not for conditions so soft from rain that balls plugged where they landed.
For now, he was glowing over another victory that keeps him as relevant as ever.
"It's nothing like the course we'll see, " Mickelson said. In 2018, Ted Potter, Jr. prevailed over Phil Mickelson, Jason Day and two-time Pebble Beach victor Dustin Johnson.