[1] The Toronto native was chosen 102nd overall by the Pittsburgh Penguins in the 1977 NHL Amateur Draft, while he was in the midst of a fine Ontario Hockey Association (OHA) season with the Sault Ste.

During the 1984–85 season he was involved in a blockbuster trade that sent him to the St. Louis Blues with Mark Johnson for goaltender Mike Liut and forward Jörgen Pettersson.

[4][5] In St. Louis, Millen formed a solid goalkeeping tandem with Rick Wamsley and helped the club reach the Conference Finals in 1986, where they were defeated in seven games by the Calgary Flames.

The following year netminder Vincent Riendeau began taking a bigger share of the starts and with college free agent Curtis Joseph in the wings, Millen was being pushed out.

On December 13, 1989, he was packaged with centre Tony Hrkac and traded to the lowly Quebec Nordiques in exchange for offensive defenseman Jeff Brown.

"[7] With Cloutier as Belfour's backup, and a Dominik Hašek now in the Chicago system, Millen was not only limited to just 58 minutes of game play in the entire season, but also his stint as a Blackhawk was over.

On October 7, the Rangers signed a new deal with Richter and Millen was assigned to the San Diego Gulls of the International Hockey League (IHL).

At the start of the year they had acquired Millen's former teammate Vincent Riendeau, who then injured his knee in his Detroit debut which sidelined him long term.

With Cheveldae carrying a heavy load, Millen was used sparingly but made the last ten appearances of his career in the Red Wings net before hanging up his pads for good at the end of the season.

During his eleven seasons affiliated with the Senators, his play-by-play partners included Don Chevrier, Rob Faulds, and Dean Brown.

After a year with CTV Sportsnet, he rejoined HNIC in 1999–2000 as the colour commentator of the network's secondary broadcast team, first paired with Chris Cuthbert and then Jim Hughson, mostly covering the second game of weekly doubleheader and three playoff rounds.

Millen is married and has four children,[21] including a son, Charlie, a goaltender, who most recently played for the Orlando Solar Bears of the ECHL.