Born in Oxnard, California, he batted and threw right-handed, stood 6 feet 3 inches (1.91 m) tall and weighed 190 pounds (86 kg).

He was awarded the starting third base job out of Spring training 1963, but after committing five errors while batting just .205 with one home run and three RBIs through April, he was optioned to triple A Spokane.

After Manager Walter Alston shifted left fielder Tommy Davis to third,[2] and tried several other players at third base, McMullen was brought back up from Spokane at the end of June.

At the end of the season, he was traded with Frank Howard, Phil Ortega and Pete Richert to the Washington Senators for John Kennedy and Claude Osteen.

[6] On September 26, 1966, he set an AL record with eleven assists from third against the Boston Red Sox (a mark later tied by Mike Ferraro).

On July 17, 1971, McMullen scored from third on a sacrifice bunt by Bruce Christensen, giving his rookie teammate his first major league RBI.

On November 28, 1972, McMullen was part of a blockbuster trade, as he and Andy Messersmith were sent to the Los Angeles Dodgers for Billy Grabarkewitz, Frank Robinson, Bill Singer, Mike Strahler and Bobby Valentine.

Robbed of the opportunity to play in the 1963 World Series by an injury, McMullen returned to the post-season in the twilight of his career with the Los Angeles Dodgers.