MARION — Former FBI director Robert S. Mueller III urged a tent full of high school graduates in this seaside town Monday afternoon to go out into the world with integrity because without that nothing matters.

Mueller, 72, whose granddaughter was one of 137 students who graduated from Tabor Academy, was recently appointed by the Justice Department as a special counsel to oversee the investigation into President Trump’s campaign and possible collusion with Russian officials.

Mueller did not mention Trump or the investigation in his 15-minute speech. Instead he encouraged the teenagers, and everyone present for the ceremony, to lead lives of character.

“We must never, ever sacrifice our integrity,’’ Mueller said as he concluded.

The decorated Marine Corps veteran of the Vietnam War has strong Boston ties. Mueller came to the city in 1982 as an assistant US attorney, later leading the office before working at Boston law firms and the Justice Department in other capacities.

Having built a reputation as an ethical, apolitical prosecutor over the decades, Mueller continued his career in law enforcement as FBI director from 2001 — starting a week before the terrorist attacks of Sep. 11 — to 2013. After retiring from the FBI, Mueller rejoined WilmerHale, a law firm he had worked for in Boston, at their Washington office.

Standing on stage Monday inside a heated tent on the school’s grassy lawn, Mueller spoke about four values he learned as Marine and while leading the FBI: public service, integrity, humility, and patience.

He told how a college lacrosse teammate inspired him to join the Marines, how his wife taught him to be more patient, and how a selfless FBI colleague kept him humble. Mueller told the students to find ways to give back, no matter where their lives lead.

“The way in which you choose to serve is not that important, only that you work to better your country and your community,’’ he said.

Mueller spoke slowly, pausing for emphasis. He said he has been surprised about where life has taken him.

It’s not what you, do, Mueller told students, but how you do it.

“You can be smart, aggressive, articulate, indeed persuasive,’’ Mueller said “but if you are not honest, your reputation will suffer and once lost, a good reputation can never, ever be regained.’’

When he spoke about patience, Mueller told a story about his wife, who taught him to be a better listener. One evening as she told him about her long day, he treated her story with the brusqueness he used in the office.

“I am your wife, I am not one of your attorneys. You will sit there and you will listen until I am finished,’’ he said she told him, drawing laughter from the graduates.

Last, he talked about humility. Mueller described a naturally humble college classmate whom he asked to be his close advisor at the FBI and who weathered the 9/11 aftermath by his side. When Mueller became stressed, he would turn to that friend.

“Don’t take yourself too seriously,’’ the friend would say. “Because no one else around here does.’’

Mueller’s granddaughter, Campbell Donley, introduced her grandfather at the ceremony on the school’s campus that overlooked a bay crowded with sailboats moving gently in the wind under cloudy skies Monday. Tuition for boarding students at Tabor is $60,000, and $43,000 for day students.

He was chosen by the school to speak at graduation before he was named special counsel for the Russia investigation. His appointment to lead the investigation came after Trump abruptly fired FBI Director James Comey.

Head of School John H. Quirk said in his remarks that Mueller was selected as speaker because of his irreprehensible character and selflessness. He thanked him for keeping his commitment to speak in light of his recent assignment to the investigation.

Students said they were impressed to learn more about someone they had known simply as the grandfather of their classmate.

“He’s got a lot of character and I respect him,’’ said Nick Stasinos, 18, of Ipswich, adding that he liked hearing about his military service.

Even though Mueller is in the spotlight because of the Trump investigation, Martha Peppes, 18, said it will be the four values he discussed that she will remember.

“It was nice that he didn’t talk about politics that much,’’ said Peppes, of Lincoln, R.I.

Tabor Academy physics teacher David Pierce said he has attended 44 commencement services. This one will stand out.

“I’m hoping that the students will listen,’’ Pierce said. “Integrity is so important. Once that’s lost, it’s gone.’’

Globe corespondent Nicole Fleming contributed to this report. Laura Krantz can be reached at laura.krantz-@globe.com. Follow her on Twitter @laurakrantz.