The 20-year-old's initial agreement was due to expire after moving from Old Trafford to the Championship outfit on deadline day in January.
However, the club have now revealed on their official website that Johnstone will remain at the Keepmoat Stadium as Ross Turnbull continues his recovery from injury.
Johnstone has made five first-team appearances.
Restore Data
| Team | P | W | D | L | F | A | GD | PTS | |
| 1 | Arsenal | 34 | 24 | 5 | 5 | 82 | 26 | 56 | 77 |
| 2 | Manchester CityMan City | 33 | 23 | 7 | 3 | 80 | 32 | 48 | 76 |
| 3 | Liverpool | 34 | 22 | 8 | 4 | 75 | 34 | 41 | 74 |
| 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 | 33 | 11 | 11 | 11 | 52 | 54 | -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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