Defeating archrival Loyola, 36-0, at Memorial Stadium in 1964, his senior year.
"We smoked them," Mulligan said. "I still hate Loyola. Anyway, I remember standing on that field, looking around and thinking, 'This is the last time I'll be here.'"
It wasn't. As a pro, Mulligan returned to Memorial Stadium three times to play the Colts, first in 1972 as a starter for the St. Louis Cardinals.
"Fantastic," he said of that day. "I got a special introduction from [public address announcer] Vince Bagli, nice applause and a message on the scoreboard that read, 'Welcome back, Number 50.' I was a Cardinal again and the first player introduced — and to play against my idols was surreal. After we won the game, 10-3, I had my picture taken with Johnny Unitas. Still have it on the wall of my weight room."
Now 68, Mulligan lives on Seabrook Island, S.C., in a home not 200 yards from the ocean. A fitness buff, he sprints on the beach and boogie-boards in the water, somewhat unusual for a grandfather of eight. Last spring Mulligan returned to Clemson, which he'd helped win two Atlantic Coast Conference championships and, despite his age, participated in an alumni flag football game.
"We had our own lockers, uniforms, announcers and referees," he said. "I was the oldest one there; most guys were in their 20s. They thought I was crazy until I stripped the flag from a cornerback who'd intercepted a pass and was running for a touchdown. Just being on that field psyched you up; it's a fire that never goes out."
Has it been 51 years since Mulligan led Calvert Hall (7-2) to the Baltimore Catholic League title?
"Of all the coaches I had, none were better than those in high school," he said. "Fred Kern, Gerry Gray, Denny Cox and Jerry Schmidt were way ahead of the curve in terms of technique and discipline. The 1960s were a wonderful period in my life. They were the beginning of the unraveling of our culture, but a great time to be at Calvert Hall."
He also played lacrosse and, as an attackman in 1965, scored five goals (plus two assists) in a 14-2 rout of Towson's defending Baltimore County champions. But football sent Mulligan to Clemson, though he had doubts once he got there.
"At 200 pounds, I competed with five other centers, the biggest of whom, Igor Umansky, was a 255-pounder from western Pennsylvania," he said. "I cried, 'Mom and Dad, take me home, I'll never play again.' But Igor flunked out and I became a starter as a sophomore."
An eighth-round pick in the 1969 NFL draft, Mulligan played five years in St. Louis and two with the New York Jets before injuries shut him down.
"I didn't run out of talent; I ran out of luck," he said. "I played against a lot of the great ones. Some years ago, at a golf tournament in Baltimore, I walked up to Willie Lanier and Bob Lilly and said, 'If you don't know me, you should because I'm the reason you're in the [Pro Football] Hall of Fame.' "
A business consultant, Mulligan still returns to Baltimore, as he did last fall for the traditional Calvert Hall-Loyola game. Nearly 20 years ago, he took part in an alumni lacrosse game between the schools and nearly caused a stir.
"A kid of about 28 was beating on me [on the field] and we almost got into a fight," Mulligan said. "I told the referee, 'Tell him to stop or one of us isn't walking off the field — and I'm walking off the field.'"
The scuffling stopped.