The former St Johnstone trainee has made over 100 appearances in Scottish football in loan spells at Stranraer, Arbroath and Elgin, but the majority came during a three-year stay at the Doonhamers.
United manager Jackie McNamara said: “Mark is a strong centre-half, something I have been looking for. He has the hunger and desire to succeed at Dundee United.
“He had greater financial offers from elsewhere but wanted to enhance his football development. He sees the opportunities for young players at Tannadice. This is where he wants to be and showing this commitment to United pleases me.
“He is experienced for his age and that fits the criteria of exactly what we are looking for. His style of play will appeal to our fans and he also pitches in with goals, which is another vital component of modern defenders. I am delighted we have secured Mark’s signature.”
Durnan said: “I have enjoyed my times at Queen of the South but I am ready to step up into the Ladbrokes Premiership and there is no better place to enhance your development than at Dundee United.
“There were other Premiership clubs interested in me but once I knew Jackie wanted me, my mind was made up. I cannot wait to meet the rest of the squad and show United fans what I will bring to the club.
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