[5] In December 1965, Pappas and another pitcher, Jack Baldschun, and outfielder Dick Simpson were traded to the Cincinnati Reds for superstar and future Baseball Hall of Famer Frank Robinson.

In 1967 Pappas won a team- and career-high 16 games, but when he got off to a slow start in 1968, he was dealt along with Bob Johnson and Ted Davidson by the Reds to the Atlanta Braves for Tony Cloninger, Clay Carroll and Woody Woodward on June 11, 1968.

Five days later, against the Montreal Expos at Jarry Park, Pappas was again part of baseball history, albeit on the other side, as he was responsible for Ron Hunt's 50th hit by pitch of the season, which broke the single-season record of 49 set by Hughie Jennings in 1896.

He retired the first 26 batters and was one strike away from a perfect game with a 2–2 count on pinch-hitter Larry Stahl, but home-plate umpire Bruce Froemming called the next two pitches—both of which were close—balls.

Eleven days after his no-hitter, Pappas recorded his 200th career victory, also at Wrigley Field, defeating the Montreal Expos 6–2.

He fell one victory short of joining Cy Young and Jim Bunning as the only pitchers to win at least 100 games in each the American and National Leagues at the time when he was released by the Chicago Cubs on April 2, 1974.

On August 27, 1961, while with the Orioles, Pappas pitched a two-hit, 3–0 shutout against the Minnesota Twins at Metropolitan Stadium and helped his own cause by homering twice off Pedro Ramos.

[13] Pappas also homered off Bill Stafford in the Orioles' 1–0 victory over the New York Yankees on April 18, 1962; he was the last American League pitcher in the pre-designated hitter era to hit a home run in a 1–0 game.

On September 11, 1982, Pappas’s wife, Carole, disappeared after leaving the couple's home in the Farnham subdivision in the Chicago suburb of Wheaton.

In 1984, Tom Kokoraleis, who was convicted for the murder of Lorraine Borowski, led police to a field where Carole Pappas was allegedly buried, but searchers could not find any remains.

[15] On August 7, 1987, workers draining a shallow pond four blocks from the Pappas home discovered the car she had been driving, a white and burgundy 1980 Buick, as well as her body.

After baseball he owned a restaurant in Baltimore, Milt Pappas' Scotch & Sirloin; worked for a beer and wine distributor; and later sold building supplies.

In 1990, Pappas sold his house in Wheaton and moved with his second wife, Judi (Bloome), a teacher of special needs children, to Beecher, Illinois, with their 5-year-old daughter Alexandria.

He was seriously injured in a February 2013 single-vehicle accident in Kankakee County when he crashed into a utility pole and rolled his Jeep Cherokee, fracturing eight ribs and lacerating an ear.