Mwanyongo, 24, is the sixth signing made by boss Liam McDonald since he took over on 14 November.
He had been training under former Moors boss Marcus Bignot at League Two side Grimsby Town but has now opted to join Solihull instead.
"He was attracting attention at league and non-league clubs," said McDonald.
"Pearson is quick and skilful with loads of ability and he'll be a massive asset for us.
"He has recently been on a week's trial at Grimsby Town and impressed, He was due to go back to them but, following a chat with us, he's decided to sign a contract at Solihull."
Mwanyongo, who has proved a prolific scorer in a wide attacking role since joining Gresley in the summer, goes straight into contention for the New Year's Day trip to Chester.
The Moors, now 16th in the table, will be out to do the double, having beaten Chester 3-2 at Damson Park on Boxing Day.
Find all the latest football transfers on our dedicated page.
Solihull Moors have completed the signing of forward Pearson Mwanyongo from Northern Premier League Division One South side Gresley.