McMillan, a Heineken Cup winner in 2007 with London Wasps and former Scotland Under-21 captain, enjoyed a previous spell with Glasgow in season 2003/04, making his debut in the Celtic Cup victory over Celtic Warriors at Bridgend.
From Glasgow, McMillan joined Leeds Tykes in 2004 before heading to London in 2006.
On his return to Glasgow, McMillan, a product of the Stirling County club, said: "I'm really looking forward to joining up with Glasgow again. The Warriors have a very hungry and ambitious squad and I'm no different.
"By returning to Scotland, I'm not only looking to enhance my own international ambitions, but I thoroughly believe that Glasgow have a squad of boys that will continue to push at the top end of the Magners League and in Europe."
Warriors head coach, Sean Lineen, added: "I remember Mark from when I first started with Glasgow. He epitomises the Scottish spirit.
"Mark has always been professional in his approach and he'll be a fantastic addition to our team. I see him as a real leader."
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