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 | Arsenal | 34 | 24 | 5 | 5 | 82 | 26 | 56 | 77 |
| 2 | Liverpool | 34 | 22 | 8 | 4 | 75 | 34 | 41 | 74 |
| 3 | Manchester CityMan City | 32 | 22 | 7 | 3 | 76 | 32 | 44 | 73 |
| 4 | Aston Villa | 34 | 20 | 6 | 8 | 71 | 50 | 21 | 66 |
| 5 | Tottenham HotspurSpurs | 32 | 18 | 6 | 8 | 65 | 49 | 16 | 60 |
| 6 | Manchester UnitedMan Utd | 33 | 16 | 5 | 12 | 51 | 50 | 1 | 53 |
| 7 | Newcastle UnitedNewcastle | 33 | 15 | 5 | 13 | 69 | 54 | 15 | 50 |
| 8 | West Ham UnitedWest Ham | 34 | 13 | 9 | 12 | 54 | 63 | -9 | 48 |
| 9 | Chelsea | 32 | 13 | 8 | 11 | 61 | 57 | 4 | 47 |
| 10 | Bournemouth | 34 | 12 | 9 | 13 | 49 | 60 | -11 | 45 |
| 11 | Brighton & Hove AlbionBrighton | 32 | 11 | 11 | 10 | 52 | 50 | 2 | 44 |
| 12 | Wolverhampton WanderersWolves | 34 | 12 | 7 | 15 | 46 | 54 | -8 | 43 |
| 13 | Fulham | 34 | 12 | 6 | 16 | 50 | 54 | -4 | 42 |
| 14 | Crystal Palace | 34 | 10 | 9 | 15 | 44 | 56 | -12 | 39 |
| 15 | Brentford | 34 | 9 | 8 | 17 | 52 | 59 | -7 | 35 |
| 16 | Everton | 34 | 11 | 8 | 15 | 36 | 48 | -12 | 33 |
| 17 | Nottingham ForestNott'm Forest | 34 | 7 | 9 | 18 | 42 | 60 | -18 | 26 |
| 18 | Luton TownLuton | 34 | 6 | 7 | 21 | 47 | 75 | -28 | 25 |
| 19 | Burnley | 34 | 5 | 8 | 21 | 37 | 69 | -32 | 23 |
| 20 | Sheffield UnitedSheff Utd | 34 | 3 | 7 | 24 | 33 | 92 | -59 | 16 |
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