The left-back has signed a two-year deal at the club with an option of a further 12 months.
Manager Lee Johnson said: "He's got a lot of potential. We'll give him an identity and a bit of love and I feel he can be a really good player for us."
Mills spent the beginning of last season on loan with the Latics, making 14 appearances.
The former Southampton defender also spent time on loan at Shrewsbury Town last season.
Restore Data
| Team | P | W | D | L | F | A | GD | PTS | |
| 1 | Sunderland | 9 | 6 | 1 | 2 | 18 | 8 | 10 | 19 |
| 2 | Burnley | 8 | 5 | 2 | 1 | 14 | 4 | 10 | 17 |
| 3 | Leeds UnitedLeeds | 9 | 4 | 4 | 1 | 15 | 7 | 8 | 16 |
| 4 | Sheffield UnitedSheff Utd | 8 | 5 | 3 | 0 | 10 | 3 | 7 | 16 |
| 5 | West Bromwich AlbionWest Brom | 8 | 5 | 1 | 2 | 12 | 6 | 6 | 16 |
| 6 | Blackburn RoversBlackburn | 8 | 4 | 3 | 1 | 14 | 9 | 5 | 15 |
| 7 | Middlesbrough | 8 | 4 | 2 | 2 | 9 | 5 | 4 | 14 |
| 8 | Watford | 8 | 4 | 1 | 3 | 12 | 13 | -1 | 13 |
| 9 | Hull City | 8 | 3 | 3 | 2 | 12 | 9 | 3 | 12 |
| 10 | Norwich CityNorwich | 8 | 3 | 3 | 2 | 12 | 10 | 2 | 12 |
| 11 | Oxford UnitedOxford Utd | 8 | 3 | 2 | 3 | 12 | 10 | 2 | 11 |
| 12 | Swansea CitySwansea | 8 | 3 | 2 | 3 | 8 | 6 | 2 | 11 |
| 13 | Bristol City | 8 | 2 | 4 | 2 | 9 | 13 | -4 | 10 |
| 14 | Stoke CityStoke | 8 | 3 | 0 | 5 | 10 | 12 | -2 | 9 |
| 15 | Derby CountyDerby | 8 | 3 | 0 | 5 | 10 | 12 | -2 | 9 |
| 16 | Millwall | 8 | 2 | 2 | 4 | 12 | 11 | 1 | 8 |
| 17 | Coventry CityCoventry | 8 | 2 | 2 | 4 | 9 | 11 | -2 | 8 |
| 18 | Plymouth ArgylePlymouth | 8 | 2 | 2 | 4 | 8 | 12 | -4 | 8 |
| 19 | Sheffield WednesdaySheff Weds | 8 | 2 | 2 | 4 | 9 | 14 | -5 | 8 |
| 20 | Luton TownLuton | 8 | 2 | 2 | 4 | 8 | 13 | -5 | 8 |
| 21 | Preston North EndPreston | 8 | 2 | 2 | 4 | 7 | 12 | -5 | 8 |
| 22 | Queens Park RangersQPR | 8 | 1 | 4 | 3 | 9 | 14 | -5 | 7 |
| 23 | Portsmouth | 8 | 0 | 4 | 4 | 8 | 19 | -11 | 4 |
| 24 | Cardiff CityCardiff | 8 | 1 | 1 | 6 | 3 | 17 | -14 | 4 |
| > Championship Full Table | |||||||||
