The 50-year-old - who has won two league titles as a manager, including the Conference Premier with the Stags - moves to the Jakemans Community Stadium from National League North rivals Kettering Town, who he has guided to eighth in the table, a point behind the Pilgrims with a game in hand.
Cox has also previously managed Eastwood Town, Torquay United, Barrow and Guiseley as well as Kettering.
Cox won the 2012-13 Conference Premier with the Stags and the 2008-09 Northern Premier League title with the Badgers.
His number two while at Mansfield was former United boss Adam Murray.
Cox fills the gap left by Craig Elliott’s departure earlier this month, and will be a spectator at today’s match at Spennymoor Town, with interim boss Paul Green leading the team.