Richard Berthelsen walked into the NFL Players Association as a young lawyer in 1972, thinking he’d stick around for three years and then maybe return to Wisconsin. Back then, the entire staff consisted of the executive director, two secretaries and Berthelsen, the newly-hired general counsel.

“I had this notion I would go to Washington, help win free agency for the players and then go back. How naive was that?” Berthelsen asked. “We did win free agency, but it was 20 years later.”

Now, 40 years after he started, Berthelsen, 67, is retiring from the players union, a move that marks the end of a historic run that gave him a front-row seat as football exploded from a Sunday pastime to the nation’s most popular sport. He was never the public face — he’d never want that — but he pulled levers and pushed buttons behind the scenes of labor battles big and small.

“It seems like randomly things pop back into my mind, things I’d forgotten about, old stories,” Berthelsen said one recent afternoon in his office. “It’s been a tremendous ride.”

Today the NFLPA has more than 100 full-time employees and takes up four full floors of office space on 20th Street NW. “We have entire departments in this building devoted to what used to be a small part of my job description back then,” Berthelsen said. Group licensing, for example, has three dozen dedicated employees. Berthelsen used to do it alone.

Finding precedents was always easy for Berthelesen because he played an integral part in all the landmark cases that involved the union. People from both sides of the negotiating table said Berthelsen was equally adept at debating and bargaining on divisive issues.

“You might disagree with him and you might be extremely frustrated by that disagreement, but he was always civil,” said former NFL Commissioner Paul Tagliabue. “He was never the type to alienate people. Dick was very articulate, but he could be unyielding until he got a chance to get together with Gene [Upshaw, the longtime executive director of the NFLPA] and identify what a good compromise might be.”

The NFL Players Association technically started in 1956, but four decades ago, John Mackey, the Baltimore Colts’ great, and the players were looking to build it into something more formidable. They hired Ed Garvey, a lawyer from Wisconsin, who quickly realized the task in front of him was bigger than one person could handle. Former Redskin Pat Richter urged Garvey to hire a general counsel, and they soon settled on Berthelsen.

“And he lasted all those years,” Richter said. “That just doesn’t happen in a political environment like professional sports is today. That, to me, says more about what he’s done and what he’s meant than anything.”

Today, players and owners share a $9 billion pie, which no one would have imagined in 1972. There was no free agency then, player safety wasn’t a discussion topic and communication between union members was lacking.

“We were creating something out of nothing,” said Garvey. “It was a paradise for a bright lawyer with a curiosity like Richard.”

Berthelsen learned quickly that winning free agency and returning home to Wisconsin to practice law would be no easy task.

“That first year on the job I remember a meeting where we were trying to reach a compromise on a pending case,” Berthelsen said, “and one of the owners was sitting in the office, gazing out the window, saying, ‘The NFL never settles.’ ”

It took strikes, lockouts and many courtroom battles — Berthelsen was always behind the curtains — but the players’ lot kept improving.

The stories about how it happened surround him in his office. That framed courtroom drawing? That was the all-female jury from the McNeil v. NFL case in 1992 that revamped free agency rules. The framed checks near the door? That is the largest back-pay award the NFLPA has won, on behalf of a player who was cut amid the 1987 strike after shaking hands with the opposing team as a gesture of solidarity before a game. The football encyclopedia that props up his laptop? That was used to settle endless office debates.

Closer to Berthelsen’s desk are the funeral service program, photograph and patch, all framed together, of Gene Upshaw, the longtime executive director of the NFLPA who died suddenly in 2008, just three days after he was diagnosed with pancreatic cancer.

Upshaw and Berthelsen had worked together since 1974, when Berthelsen was still a young lawyer and Upshaw a union player representative for the Oakland Raiders. For 25 years, the two helped guide the union together. When Upshaw died, Berthelsen was named interim director and began working immediately to find a permanent replacement.

“It was almost like I couldn’t stop to grieve and deal with it in a normal way,” Berthelsen said.

The NFLPA settled on a trial lawyer named DeMaurice Smith, who joined the union in 2009 on the cusp of a lockout and major labor battle. Berthelsen prepped the new executive director in what Smith calls “hour-by-hour, minute-by-minute shepherding.”

“There was no person more indispensable than Richard,” Smith said. “You learn rather quickly that not only is he a brilliant lawyer, but he understands better than anybody else the cycle, the process, the paradigm that we’re in.”

Berthelsen did not have the luxury of specializing in one area of the law. Smith said the required areas of expertise include labor law, workers compensation, antitrust issues, negotiations, public relations, grievances, player membership, the NFL and its owners and dozens of subsets of each category.

“In sports, there are only a handful of people who have been able to do all that over a long arc of history,” Smith said. “You can almost name them on one hand: Don Fehr, Marvin Miller and Richard Berthelsen.” Fehr and Miller were the longtime union officials for Major League Baseball players. Like Berthelsen, sport changed and evolved dramatically on their watch.

Michael Weiner, executive director of the baseball players association, calls Berthelsen “one of the giants.

“His understanding of the law, his understanding of the bargaining process, his understanding of what makes unions tick — he really is second to none,” Weiner said. “They’re going to miss him.”

Someone else eventually will assume the role of NFLPA general counsel, but as Smith said, “We’ve made the decision that we’re not replacing Richard.”

Tuesday is Berthelsen’s last day, and at some point, he’ll have to pack some boxes. The NFLPA will set aside a smaller office for him. Berthelsen will pop in now and then and will talk regularly with Smith.

He’d like to sit down and do some writing about all that he’s seen and done these past 40 years. He’s had a rare seat from which he watched the evolution of the sport — and the union — and is uniquely qualified to document their growth. “I’d say uniquely obligated,” Berthelsen said.

Forty years ago, he thought he’d be in Washington for only a couple of years. Now, as he prepares for what he promises will be a busy retirement, Berthelsen can’t imagine having spent the past four decades doing anything else.

“I had a professor in college who said you don’t really have a full professional life unless you have seven or eight different jobs,” Berthelsen said. “I ascribed to that. I thought that made sense. But I’ve had the same job for 40 years. It really has been in a lot of meaningful ways, at least a dozen different jobs. And I’ve enjoyed them all.”