Nationals principal owner Mark Lerner is recovering from spindle cell sarcoma in his left leg, requiring the limb to be amputated. (Jonathan Newton/The Washington Post)

Regardless of where the Washington Nationals sit in the standings, who’s on the mound or who’s in the lineup, there has long been a night-to-night constant at Nationals Park: Mark Lerner, one of the club’s principal owners and the only son of family patriarch Ted, sitting in the first row, Nationals hat on his head, monitoring the action from the on-deck circle to the outfield seats. He is the face of the family that owns the club, and its most visible presence at the ballpark.

But over the past several months, as the Nationals have surged toward what will likely be their fourth division title in six years, Lerner’s front-row perch has sat empty, conspicuously vacant. Season-ticket holders asked, reporters asked, players asked: Where’s Mark?

“There’s been a void there,” Nationals General Manager Mike Rizzo said.

Thursday night, in a letter to The Washington Post, Lerner revealed the reason for his absence: Lerner had cancer in his left leg, and though he is now cancer-free, complications following surgery and radiation treatment left doctors with little choice but to amputate the leg last week.

“With my doctors and medical team, we decided that amputation of that leg was my best choice to maintain the active and busy lifestyle that I have always enjoyed,” Lerner said in his letter, relayed through the Nationals public relations department. “The limb was removed in early August and I’m healing well, cancer-free, and looking forward to my eventual new prosthetic.”

The absence of Lerner, 63, from Nationals Park was stark. Few owners are as visible on a nightly basis. Rarely is he hidden in a luxury suite, and viewers of the team’s games on the Mid-Atlantic Sports Network could easily identify Mark Lerner seated in, as Rizzo said, “Row 1, Seat 1,” nearest the Nationals’ dugout during home games.

But as this season began, Lerner was infrequently in that seat. The last game he attended was July 30, but he was barely present in the months leading up to that.

“I know you recognize that only something really challenging would have kept me from my favorite seat at the ballpark these past months,” Lerner wrote.

Lerner and his family, who have a reputation for being intensely private, kept the information tightly under wraps. Senior staff were informed only during a homestand that just concluded Wednesday. Rizzo told the team Thursday night after batting practice in San Diego, where the Nationals opened a seven-game trip.

“It’s been difficult to explain to players, to staff, to everybody,” Rizzo said by phone. “They want to be private about it, and that’s their right. But his absence was noticed. Not having him in the draft room; he’s always in the draft room. Not having him at the trade deadline, when you’re talking to Mr. Lerner and the group 20 times a day. I know they were keeping him abreast, and he had other things on his mind, but I missed him. It was sad. I was bummed out about it.”

In his letter, Lerner said in early January doctors discovered a rare type of cancer called spindle cell sarcoma in his left leg, above the knee. According to the National Institutes of Health, sarcomas are malignant tumors that attack connective tissue and are most common in bones, muscles, tendons or cartilage — particularly in arms and legs.

Lerner said he underwent radiation, which was completed in March. In April, he had surgery that successfully removed the cancer. But there were problems thereafter.

“The radiation treatment eventually caused the wound not to heal properly,” Lerner wrote.

With that, Lerner, his family and the doctors worked through options, eventually determining that amputation — above the knee — was best. Despite that, Lerner said he expects to be able to return to a lifestyle similar to what he enjoyed before he lost the limb. When his family first purchased the Nationals in 2006, Lerner distinguished himself from other owners by frequently suiting up in full uniform and shagging fly balls during batting practice. While he abandoned that practice in recent years, he is an avid golfer, and once he is fitted with and becomes accustomed to a new prosthetic, he expects to be able to continue with that pursuit, as well as others.

The Nationals say Lerner will not be limited in his duties either with the club or the family’s real estate business. Details about his rehabilitation — both a timeline, and where it will take place — haven’t been worked out. He is not yet home, but expects to return soon.

“He’s a fan, of course, but it’s more than that,” Rizzo said. “He has a deep-rooted interest in this team — not only a business interest, but much more importantly, he feels that he’s the caretaker of the team in Washington, D.C., his hometown. We all want to win a World Series. I want to do it for D.C. and the area. But it’s probably multiplied double for Mark. That’s his town. He grew up there. It’s his family. It’s a big deal.”

Mark Lerner is the oldest of Ted and Annette Lerner’s three children. He and his two sisters, Debra Lerner Cohen and Marla Lerner Tanenbaum are joined by his brothers-in-law, Robert Tanenbaum and Ed Cohen, and his Mark’s wife, Judy, as principal owners of the Nationals. Ted Lerner, 91, is still actively involved in major decisions regarding the club, but the family rules by consensus, voting on each issue.

That close relationship, Mark Lerner said, will help him get through a difficult challenge.

“I’ve been very blessed with my wonderful wife Judy, who has never left my side, our great kids, amazing family and close friends,” Lerner wrote. “I really appreciate everyone respecting our family’s privacy as we’ve gone through this. I’m not sure of the timeline yet, but you know I’ll be at Nationals Park as soon as I possibly can.”

Chelsea Janes contributed to this report.

Read more:

Read the letter from Mark Lerner to The Post regarding his bout with cancer

Strasburg primed and ready to go

Nats are beaten up but sailing along