That possibility was nullified last Saturday when Johnson lost to the Mets, leaving him with 298 career victories. He notched 130 of those wins during his formative years with the Mariners, from the middle of the 1989 season to the middle of the '98 season.
He pitched his first no-hitter, won his first Cy Young Award and appeared in the postseason for the first time as a Mariner.
Now, Mariners fans have a chance to see The Big Unit at least one more time as he gets ever closer to becoming the 24th pitcher in Major League history to win 300 games.
"It would have been more dramatic for him to be going for No. 300," Hall of Fame broadcaster Dave Niehaus said of Friday night's Interleague Play series opener against the Giants, "but just to have him come back and pitch here, probably for the last time, will be a big night."
The three-game series kicks off the Interleague Play season, but Friday night's opener is mostly about Johnson and his return to Seattle.
Niehaus was in the booth for most of Johnson's victories with the Mariners.
"He probably has two signature games," Niehaus said. "The no-hitter against Detroit, and I can still see the Tigers catcher [Mike Heath] swing at a pitch over his head for the final out. And coming out of the bullpen in Game 5 of the '95 playoffs against the Yankees."
Those memorable events figure to make it a warm welcome for the 6-foot-10 future Hall of Famer, who will be making the 595th start of his big league career -- and his fourth at Safeco Field.
He is 2-1 with a 1.50 ERA in three previous starts at Safeco, and the lineup he faces Friday night includes players that have done well against in him the past (Ichiro Suzuki is 7-for-15 and Jose Lopez is 4-for-10), and some that have not done so well (Mike Sweeney is 3-for-25 and Adrian Beltre is 14-for-62 with 18 strikeouts).
"I remember facing him when his slider was faster than most guys' fastballs," Sweeney said. "The first time I faced him, I saw a 97- to 100-mph fastball and his slider was 91. He was head and shoulders, both in stature and stuff, above anyone else in the game."
The slider had a name, "Mr. Snappy", and it was Johnson's great equalizer against right-handed batters.
"It would eat your feet alive," Niehaus said. "Hitters would foul that pitch off their foot on one of those sliders and the same thing would happen on the next pitch. He used to kill feet."
Sweeney has been the Mariners' designated hitter in most games against left-handers this season, so it figures that he will be in the lineup Friday night.
"It will be neat for us to face him," he said, "and for him to come home to a place where he pitched for so many years. It will be good for the fans, too."
Johnson is only a shadow of his once-dominating self. He is 3-4 with a 6.86 ERA this season.
"The radar readings aren't as high," Sweeney said, "but he's still good."
The Big Unit is one of the best in history.
"Of the pitchers I have had a chance to describe," Niehaus said, "Randy is the most dominant pitcher I ever saw, except for [Sandy] Koufax during that four- or five-year stretch in the 1960s. I'm really looking forward to Friday night's game, although this isn't the same Randy Johnson that we saw 10 or 15 years ago."
Mariners assistant general manager Lee Pelekoudas said Johnson's start in the Interleague Play series opener will be a "special event" for the fans who are expected to fill the place he helped build.
"He did a lot of things for the organization," Pelekoudas said," and was a big part of the '95 and '97 [playoff] teams."
Johnson has fond memories of his Seattle years.
"That was pretty much where my whole foundation was," he said. "I pitched every fifth day. It was sink or swim. I had that opportunity. That's really kind of where everything came together for me. That's really where I learned how to pitch.
"Most people don't realize this, but in my first seven years in the Major Leagues, I played for teams that didn't even finish over .500. The point being, had that been a team that wasn't finishing below .500, I might not have had that opportunity to sink or swim," Johnson said.
"Everybody on that [Seattle] team at the time -- Omar Vizquel, Edgar Martinez, Jay Buhner, Ken Griffey Jr. -- we were all kind of learning at the same time to play in the Major Leagues. We had that opportunity and it was afforded to us. The bulk of the team was very young, myself included, and we had the opportunity to learn by our mistakes."
Johnson dominated hitters back then, but not so much anymore. He is now 45 years old and possibly in the final chapter of a brilliant career. The radar guns clocking his fastball rarely reach the low 90s these days, compared to the high 90s in his prime years.
But there are occasions when he still flashes that Big Unit of old form. Perhaps that will show up one more time at Safeco Field, but the Mariners would just as soon have that happen the following week when Johnson faces the Braves at AT&T Park in San Francisco.
Friday night is mostly about turning back the clock to the 1990's, when Johnson reached double-digit strikeouts almost every game he pitched at the Kingdome.
Those days are long gone, but the memories will last forever.
Many of those memories will return on Friday night.