Former Tranmere defender McChrystal, 28, put pen to paper on a new two-year contract with Rovers on Tuesday.
The player initially joined on a short-term deal following his release from Prenton Park.
"We are delighted that Mark has committed himself to the club," McGhee told his club's official website. "We are aiming to try and improve things here, and he is an important part of the jigsaw.
"He has been part of the stable back four that have been playing together in recent months, and we are also talking to Steve Mildenhall.
"We are a young squad, and Mark gives us that seniority and experience, and I am sure Tom Parkes would be the first person to say he has benefited from playing alongside Mark."
Chairman Nick Higgs added: "We are delighted that Mark has decided to sign a new deal with us.
"He is an experienced player, who made an instant impact when he joined the club at the turn of the year.
"He helped shore up our defence, and was one of the pivotal players in turning our season around.
"We hope that his experience will help us next year as we look to push for promotion."
- Sports & Recreation