Mace, 38, is a seasoned non-league defender who joins the club from Boldmere St Michaels.
The centre-back is widely regarded for his time at Leamington, where he made 436 appearances over a 12-year period.
Since departing the Brakes, Mace has also featured Stratford Town and Bromsgrove Sporting.
Joining him at Butlin Road is young ‘keeper Nykah Liburd-Hines.
The 22-year-old, who began his career at Nottingham Forest’s Academy, departs Carlton Town to link up with Carl Adams’ side.
A former Hereford, Chippenham and Lincoln United player, he was restricted to just one appearance for the Millers and will look for more opportunities at the Valley.