John Mara, the 70-year-old president, chief executive officer and co-owner of the Giants, has recently been diagnosed with cancer.

He is undergoing treatment, according to a statement he released through the team Monday afternoon.

“I have recently been diagnosed with cancer and have been following the treatment plan recommended by an outstanding team of doctors,’’ Mara’s statement read. “I’m feeling strong and optimistic, and I’m committed to seeing this through to a positive outcome. I plan to remain active with the team throughout my treatment, and I’m fortunate to be surrounded by incredible support — personally, professionally, and medically. I ask that you respect my privacy and my family’s privacy at this time.”

Mara was in attendance at MetLife Stadium on Sunday as the Giants beat the Chargers 21-18 for their first victory of the season after losing their first three games.

Unlike many NFL owners, Mara is a constant presence in the team facility, with an office in the building. He is frequently on the field to watch practice and is in daily contact with general manager Joe Schoen and head coach Brian Daboll.

The type of cancer Mara has and the stage and severity of the diagnosis was not revealed at this time. He appears to have lost weight.

The Giants are a family business. Mara is the oldest son of the late Wellington Mara, a franchise patriarch. John Mara’s grandfather, Tim, purchased the franchise for $500 in 1925.

John Mara has a law degree from Fordham University Law School and worked as a labor and employment lawyer and litigator prior to joining the Giants in 1991.

He became the team president in 2005, following his father’s death.

In 1990, the Tisch family purchased 50 percent of the Giants for $75 million. Steve Tisch is currently the co-owner of the team, along with John Mara. Recently, investor Julia Koch and other members of the billionaire Koch family agreed to buy a minority stake of 10 percent in the Giants.

The franchise is valued at $10 billion.

Under John Mara’s direction, the Giants added two Lombardi Trophies to the glass-enclosed case at the team facility, one after the 2007 season and another after the 2011 season. Mara is widely respected around the NFL.

He was the chairman of the league’s Management Council Executive Committee, and for the past 25 years he has been an influential member of the NFL’s Competition Committee.

The franchise has fallen on hard times, with 10 losing seasons in the past 12 years and the Giants off to a 1-3 start to this season. Mara has gone through five head coaches and three general managers in the past 11 years.

After last season’s 3-14 finish in the 100th year in franchise history, Mara stated he would be retaining Schoen and Daboll but insisted on seeing improvement. “Better not take too long, because I’ve just about run out of patience,” Mara said. “I’m going to have to be in a better mood this time next year.”