Kevin Plank, founder and chief executive of Under Armour, announced a $16 million donation to alma mater St. John’s College High School in Northwest Washington. (Chris Goodney/Bloomberg News)

Under Armour founder Kevin Plank announced a $16 million gift to his alma mater, St. John’s College High School, Friday night at a gala for alumni of the private Catholic preparatory school in the District.

The donation is the largest in the 164-year history of St. John’s, school President Jeffrey Mancabelli said.

“It’s breathtaking,” he said, calling the donation “transformative.”

Mancabelli said that the donation will be used to fund a second phase of the school’s master plan to make improvements on campus, including state-of-the-art athletics facilities and a new academic program that focuses on entrepreneurship — two hallmarks of Plank’s professional career.

After Plank graduated from St. John’s in 1990, he enrolled at the University of Maryland, where he played football for the Terrapins and was a special-teams captain. In 1996, he started an athletics clothing company in his grandmother’s basement in the District that specialized in microfiber T-shirts that wicked away sweat.

Today, Under Armour is an athletics gear behemoth with a market capitalization of more than $20 billion, and Plank is a billionaire ranked by Forbes magazine this year at No. 145 on the list of America’s 400 richest people.

“I am committed to investing in education, entrepreneurship and promoting physical fitness in Baltimore and Washington, D.C.,” Plank said in a statement. “Providing great facilities, academics and programming to current and future students at St. John’s is just one very important part of that commitment.”

Mancabelli said that Plank is a consistent supporter of St. John’s, where he found sanctuary and a second chance during his turbulent teenage years.

In an interview with Forbes, Plank said he spent his first two years in high school at Georgetown Prep before he was kicked out during his sophomore year after getting drunk and fighting with members of the Georgetown University football team.

St. John’s offered him a fresh start and the opportunity for reform, setting him up to attend U-Md. Plank has offered significant sums to St. John’s and U-Md. in the years since. In 2014, he announced a $25 million gift to Maryland to renovate its Cole Field House.

“The importance of both St. John’s and the University of Maryland in Kevin’s life led him to make major commitments to both institutions, in each case on the multiple fronts of academics, entre­pre­neur­ship, athletics and, importantly, financial support for students who otherwise couldn’t have these experiences,” said Tom Geddes, executive director of Plank’s charitable organization, the Cupid Foundation.

Plank’s donation comes amid recent years of significant philanthropy to private schools in the Washington region. In April, two donors contributed $20 million to the Jewish Primary Day School in Northwest Washington to build a middle school. In 2014, the private Foxcroft School in Middleburg, Va., received a $40 million bequest from a Connecticut heiress.

Mancabelli said Plank’s gift will help the school with capital projects years ahead of schedule. In addition, the gift will be used to provide resources for a new entrepreneurial academic program that focuses on leadership for the school’s 1,100 students.

“It’s going to impact every student at the school,” Mancabelli said. “I think every student will feel the impact of his generosity.”

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