From developing together with the AHL's Hershey Bears to emerging as NHL All-Stars, much has been shared by the organizational stalwarts. When they reached the pinnacle of their sport in 2018, it was fittingly Carlson who handed the Stanley Cup to Holtby during the team procession.
And when the Capitals reconvened for training camp last month, another shared experience greeted them. One, they admit, snuck up on them.
"We were getting our physicals and medicals," Holtby recently recalled, "and John and I were in the 'Old Group.' I don't know what to make of that."
Carlson tried to take it in stride.
"It was funny," he said. "We were actually sitting and getting an EKG and they're asking how old we are and we're like, 'Oh God, 29!"
It's gone fast, but Carlson has evolved from a rising superstar to a wily veteran still capable of producing big numbers and playing heavy minutes. He's already being counted on to do both again in 2019-20. With his 10th NHL campaign underway, a case can be made that Carlson's value to the Capitals has never been greater.
"He's the heartbeat of our blueline," said forward T.J. Oshie, who trained with Carlson in the offseason. "In my opinion, he's one of the top two or three D men in the league."
Carlson had 13 goals and a career-high 70 points in 80 games last season. He also played more than 25 minutes per game for the first time in his career. Along the way, he was named an NHL All-Star. By year's end, he finished a personal-best fourth in voting for the Norris Trophy.
"He's a horse for us," said Lars Eller. "He's just as good defensively as he is offensively. I think he maybe doesn't get enough credit for how good he is defensively. He's doesn't lose a lot of battles. He's an incredibly well-rounded defenseman. We're lucky to have him."
While player turnover is inevitable in the offseason, the recent departures of veteran defensemen Matt Niskanen and Brooks Orpik leave Carlson as Washington's elder statesman on the back end. That could mean even more ice time in key situations and the need to take on more of a leadership role. Carlson is confident he's equipped for both.
"I think so," said Carlson, who was named an alternate captain before the season. "Respect is not just about age anyways. I'd like to think of myself as a leader for a long time, but certainly with the guys that left, I'll need to step up a lot more and take over collectively for them."
Carlson doesn't anticipate too much will change in how he carries himself personally, but he acknowledges that the environment around him may be different and that he'll adjust accordingly.
"Being a part of more of an elder group the last five years, there was no policing, nobody needed to say much to each other," Carlson said. "We were all pros that had been through it all before. It will be different this year."
The Capitals are confident in the core that will form their blueline this season but without Niskanen and Orpik - key cogs over the previous five seasons - several individuals will be called upon to do more heavy lifting.
Nick Jensen, in his first full year in Washington, is expected to see elevated minutes in a top-four role. Jonas Siegenthaler will look to solidify a fulltime role. Dmitry Orlov will aim to return to the form he showed during the 2018 Cup run. But if the Capitals are to win a fifth consecutive Metropolitan Division title, Carlson could be the catalyst.
"There are guys that need to step up, myself included, being here the longest and all that," said Carlson, who opened the season with two goals and 11 points through seven games. "I don't want to change who I am, but there certainly will be times where I have to think about certain things more than in the past.
"It certainly feels different. Two of the big veteran presences not only on D, but in the room, being gone, that's a big change. It needs to be met with subsequent changes."
Carlson believes he's shown leadership qualities throughout his career, but that his years of experience now provide him with even more credentials.
"I think I'm maybe a little more qualified now for people to listen to me rather than just putting in my two cents," he said.
Carlson is just beginning the second season of an eight-year extension he signed with Washington in 2018. He has grown both on and off the ice in these parts over the last decade, making it his year-round home with his wife and two children.
"We've seen him mature," said head coach Todd Reirden, who began working with Carlson in 2014 and predicted he would emerge as a Norris Trophy candidate.
"I think with some of the leadership stuff that he's going to be asked to do," Reirden said, "that starts to heighten his value around the league as well."
With Carlson's long-term future secure, and having come off the demanding (and equally as rewarding) Stanley Cup run in 2018, the 6'3", 217-pound blueliner may have been excused had he eased into last season. Instead, he showed why he's emerged as one of the League's best.
"He plays like he's in his 15th season," Oshie said, "as far as his work ethic, doing the right things, not making mistakes and cleaning them up pretty quick if he does."
By the end of the month, Carlson is set to join Calle Johansson and Rod Langway as the only defenseman to play at least 700 games with the franchise. By season's end, he could also join Larry Murphy and Scott Stevens as the only defensemen in Capitals history to record three consecutive 60-point campaigns.
"I'd just like to win again," Carlson said. "That was the best experience any player can have. As soon as we lost [last spring] that's what you focus on- this upcoming season and putting ourselves in position to win. I think we had a real productive camp; guys came back earlier in the summer than in years past. We're ready.