After a nine-game trial late in 1961, Donn was eased into the lineup the following year at first base and in right field. He hit .302 with 20 of his 67 hits going for extra bases, and swiped 16 bases. He received the single vote for N.L. Rookie of the Year that did not go to the winner, Cubs second baseman Ken Hubbs. But the young Pirate would become a steady and productive hitter for his club throughout the 1960s, increasing his RBI total each year from 1962 (28 in 222 AB) to 1966 (a career-high 98). He socked double-digit home runs for nine straight seasons, back when that meant something. In 1965 and 1966, the first baseman went even further, reaching double digits in doubles, triples, and home runs (32-14-14 and 22-10-28, respectively). As with most hitters, his production dropped at the end of the decade, but he was the leading RBI man for the Bucs in 1968 with 87.
Donn gave his "Amazin'" new team an infusion of calm veteran presence and power hitting, clubbing a dozen home runs in 72 games. But he saved his best for the World Series, batting .357 and leading the Mets with three home runs and four RBI as the all-too-recent laughingstocks of baseball shocked the powerhouse Orioles and the rest of the world. He was named Series MVP.
Not content to be defined as an ex-athlete, Donn found a new calling by earning a law degree from Duquesne University. He worked at law firms in Washington, D.C. and Chicago, and battled with drug addiction, eventually rehabilitating at a facility in Ogden, Utah. Afterward, he continued to practice law in Sioux Falls, South Dakota and became an addiction counselor. In September 2005, Clendenon passed away at age 70 after a lengthy battle with leukemia.