"I've worked with youth players right up to first-teamers, and I think I've proved myself at both clubs," he told BBC Radio Shropshire.
"I've been loyal. I had eight years at Tranmere and three at Shrewsbury.
"It's not often a club like Liverpool comes calling, so it's a fantastic opportunity for me," he added.
McMahon, who is the younger brother of former Anfield midfielder Steve, was on Everton's books as a youngster.