He played junior hockey for the Sudbury Wolves. In 63 games with the Wolves in his rookie year he produced 16 goals and 44 points along with a noticeable willingness to play a rough and tumble game, with 106 minutes in penalties.
The Quebec Nordiques selected him in the third round of the 1985 NHL Entry Draft, 57th overall. He played parts of three NHL seasons in Quebec before going to the Edmonton Oilers in the 1990–91 campaign.
After that season Middendorf played in the minor pros until he retired in 1998.
Middendorf had only played 13 games in the NHL, collecting 2 goals and 4 assists.