The 28 year-old former Leeds Tykes front-rower is renowned for his exceptional strength and powerful scrummaging ability, with the Yorkshireman becoming a key anchor in the Warriors front-row this season.
Head coach Gregor Townsend said: "To have secured the services of Mike is great news for the club. He is developing a formidable reputation as one of the best tight-heads in the game and to have him here at Scotstoun for the long-term is terrific news.
"He contributes well in defence and I've also been impressed by his handling ability.
"With Test players Moray Low, Jon Welsh and Ed Kalman also able to play tighthead, Mike knows he has to continually work hard to win his place in the side. This competition can only drive up the standards for the whole club."
Cusack, who at 28, is entering his prime as a front row said: "This was an easy decision for me to make. There is a great buzz around Scotstoun and we're building an exceptionally strong side.
"I want to help this club achieve success and that means committing to the direction the club are going.
"I know that there is always going to be competition for places here at the Warriors, but that can only improve me as a player and help the club."
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