The 30-year-old, who scored eight goals in 84 appearances for the Pilgrims between 2014 and 2016, has since played for Grimsby Town, Morecambe, Oldham Athletic, Port Vale and Walsall during a successful EFL career.
Subject to final National League and Football Association approval, Mills will be eligible to face Southport on Saturday, having been allocated squad number 23.
Reflecting on Mills' arrival, United manager Paul Cox said: "I am absolutely delighted to have Zak onboard.
"I was already relatively happy with our defensive options, but when Zak became available, it was a no-brainer to bring him in.
"He has quality, experience and versatility in abundance. I am sure he will be a great asset."
The club have also confirmed the permanent signing of Joe Leesley.
The 28-year-old has put pen-to-paper to join the Pilgrims after leaving EFL side Harrogate Town – and recovering from a knee injury picked up at AFC Telford United on April 30.
Leesley has nine goals and 19 assists to his name during two loan spells with the Pilgrims – in just 44 starts and nine substitute appearances.
His injury at the New Bucks Head deprived the Pilgrims of the player's services for the play-offs – but he is now fit and available ahead of Saturday's big kick-off against Southport.
"Joe possesses quality and experience in abundance – two vital ingredients for any side at this level," reflected United manager Paul Cox.
"He's worked hard to come back from that unfortunate injury at the end of April and I am looking forward with Joe once again this season.
"Joe is extremely versatile and can play in a number of positions."