The trio, boasting 11 green jackets between them, got the 89th Masters underway at Augusta National as ceremonial starters on Thursday morning.
They then addressed the drawn-out merging efforts of LIV Golf and PGA Tour, nearly two years since a framework agreement was tabled.
Twelve LIV Golfers, including Australian Cameron Smith, are in the field this week.
At three under, American Bryson DeChambeau and Englishman Tyrrel Hatton are the best of them after 18 holes, four shots behind standout leader Justin Rose.
Smith shot a one-under 71, a round matched by LIV compatriots Bubba Watson and Patrick Reed.
Sergia Garcia is even par while Brooks Koepka and Dustin Johnson finished two over and Mickelson, now 54, is a stroke further back after a first-round 75.
Mickelson is in jeopardy of missing the cut for the first time since 2016 and only the fourth time in 32 appearances.
Two years ago, he shot a final-round 65 to finish tied for second, the best Masters finish ever by a player 50 years or older.
Asked if he was surprised to see the left-hander still playing, Nicklaus offered a cheeky response.
"The PGA Tour is the tour, and that's where most of your good players are, and I think it's very healthy no matter which way it goes," he said.
"I don't know what level Phil is competing at. I guess he's still playing. He's playing the LIV Tour, is he?
"I don't know if he's playing or not. I don't know, you never see that anymore.
"I think the LIV pushed the PGA Tour into doing some things that were a little premature for the PGA Tour. But the PGA Tour is doing fine."
US President Donald Trump is now involved in peace talks but Watson doesn't see a way forward for the two bodies.
"They made their choice to play their own tour, and that's where they are right now," Watson said.
"I don't see a real working mechanism for the two tours to get back together."
All three had tipped Rory McIlroy to end his 11-year major curse and collect a career grand slam, Nicklaus's prophetic analysis after the Northern Irishman twice double-bogeyed on the back nine to finish even par.
"The discipline to do that is what Rory has lacked in my opinion. He's got all the shots, he's got all the game," he said, revealing they had sat down to talk through his Masters strategy last week.
"But if you go back through his history in the last few years he gets to a place and all of a sudden a seven or an eight pops up and that keeps him from getting where he needs to go."
