For 1973, Milner was moved to first base. He was leading his team with a .328 batting average, five home runs and thirteen runs batted in when he suffered a hamstring injury against the Houston Astros on April 25,[3] an injury that plagued him for the rest of his career. He returned to the club in mid-May, but went into a 12-for-82 slump that saw his batting average fall to .216. He returned to form by the end of the season to lead his team with 72 runs batted in and a career high 23 home runs as the Mets completed their improbable run to their second National League East crown.
On September 11, 1974, Milner tied a major league record by making twelve plate appearances in the Mets' 25 inning marathon with the Cardinals.[5] That season, he led his team in home runs for the third year in a row with twenty, and also led his team with seventy runs. For his career with the Mets, Milner batted .245 with 94 home runs and 338 runs batted in.
Milner found himself in more of a reserve role with Pittsburgh, with the 333 at-bats he logged in 1978 being his fewest outside of his injury plagued 1975 season. His finest season with the Pirates came in 1979, when he hit sixteen home runs and drove in sixty backing up Willie Stargell at first and Bill Robinson in left field. Milner was held hitless in nine at-bats in the 1979 National League Championship Series with the Cincinnati Reds, but was 3-for-9 in the World Series with a run batted in and two runs scored.
Milner developed and refined a reputation as a reliable pinch hitter who thrived in pressure situations during his tenure with the Pirates. On August 20, 1981, he was traded to the Montreal Expos for Willie Montañez, who was also part of the four team trade on December 8, 1977 (Montañez landed with the Mets). Milner reached the post season for the third time in his career as the Expos won the second half of the strike shortened 1981 season. He was 1-for-2 in the 1981 National League Division Series against the Philadelphia Phillies with a runs batted in during the fourth game.[8] When Milner was released by the Expos midway through the 1982 season, he rejoined the Pirates. He was released by the Pirates the following spring, and retired.
A pure fastball hitter with lightning-quick wrists, Milner posted a 1.066 walk-to-strikeout ratio (504-to-473) for his career. Milner hit ten career grand slams, including three with the Mets in 1976.
After his playing career, Milner was called to testify at the cocaine distribution trial of Curtis Strong. He stated during his testimony that he used cocaine from 1978 through 1984, and that he bought two grams of cocaine for $200 in the bathroom stalls at Three Rivers Stadium during a June 13, 1980 Pirates-Astros game, and on September 30, 1981. He also claimed to have used a liquid form of amphetamines called "red juice" he acquired from Willie Mays while both played with the Mets, and that "greenies" (also an amphetamine) were often anonymously placed in his locker while he played with the Pirates. Dale Berra and Dave Parker both attributed the greenies to Willie Stargell. However, both he and Mays denied the claims made against them.[9] Berra also claimed to have shared cocaine with Milner, Parker, Lee Lacy, Lee Mazzilli and Rod Scurry while all were members of the Pirates.