The pair will provide Danny Wilson and his coaching team with invaluable additional squad depth, as the club prepares for the conclusion of the 2020/21 Guinness PRO14 and the newly-formed Rainbow Cup.
Born in Melrose, the tight-head prop returns to Scotstoun having previously impressed for the Southern Knights in the inaugural FOSROC Super6 season.
His career path has also included a short stint with both Yorkshire Carnegie and London Scottish, having previously been part of the Elite Development Programme at Edinburgh.
"I'm really excited to get this opportunity," McQuillin told glasgowwarriors.org.
"To sign for Glasgow Warriors and to get the chance to build on my performances from last season is something I'm looking forward to.
"I've worked really hard to get to this position and I can't wait to get back in amongst the squad and competing for the jersey."
Meanwhile, hooker Matthews has agreed to a contract extension that will see him continue at Scotstoun until the end of the current campaign.
The 27-year-old initially joined the club in the summer of 2019, making his debut away to the Cheetahs in the opening match of the 2019/20 season.
Originally from Liverpool, Matthews qualifies for Scotland through his Glaswegian mother and has represented Scotland U18s and Scotland Club XV over the course of his career to date.
His first try in Glasgow colours came against Dragons at Scotstoun earlier this season, touching down in the 83rd minute from a quick tap penalty.
Speaking to glasgowwarriors.org the Scotland Club XV cap said: "I'm really happy to be able to get the chance to continue playing for this club.
"So far this season I've had opportunities to show what I can do. I feel like I've found my stride recently and I'm really looking forward to continue staking my claim for a spot in the match day 23."
Speaking on the signings of the front row duo, Head Coach Danny Wilson said: "This season has been like no other. We are likely to lose a lot of boys to international duty during the Six Nations again, and being able to have the likes of Johnny and Ewan available to us will be really beneficial.
"Johnny has been playing well for us recently and we're delighted to secure his services until the end of the season.
"We were really impressed with his scrummaging ability and we're looking for more of the same from him this time around."