The midfielder has been training with the Hatters in recent weeks after leaving Hull City in the summer.
Mainwaring first joined the Hatters in 2008 after impressing on trial whilst still at Preston.
After playing twenty-two times in his first season, he unfortunately suffered a broken leg during pre-season in 2009, keeping him out for the whole of 2009/10.
After being released in September 2011 he joined Championship side Hull City, then returned to Edgeley Park on loan last January, turning out fifteen times as County finished the season strongly.
Mainwaring said: "A new manager came in at Hull and decided to bring his own lads in, so I went out to Scunthorpe on trial. But their centre-half got injured so they made it their priority to bring a defender in.
"I contacted Jim about coming in, training and keeping ticking over, then we had a chat and I signed on a non-contract basis while I'm in between clubs. But obviously I want to play a part. Every part, if I can!"