The 36-year-old was a free agent after leaving Notts County at the end of last season.
Mahon becomes the League Two side's third signing of the week, following the arrivals of goalkeeper Trevor Carson and midfielder Therry Racon.
"I'm really happy to have signed for Pompey," Mahon told BBC Sport. "I can't wait to get started and I'm looking forward to my time here."
Mahon made 49 appearances for Notts County.
Restore Data
| Team | P | W | D | L | F | A | GD | PTS | |
| 1 | Sunderland | 9 | 6 | 1 | 2 | 18 | 8 | 10 | 19 |
| 2 | Sheffield UnitedSheff Utd | 9 | 6 | 3 | 0 | 12 | 3 | 9 | 19 |
| 3 | Burnley | 9 | 5 | 3 | 1 | 14 | 4 | 10 | 18 |
| 4 | West Bromwich AlbionWest Brom | 9 | 5 | 2 | 2 | 12 | 6 | 6 | 17 |
| 5 | Leeds UnitedLeeds | 9 | 4 | 4 | 1 | 15 | 7 | 8 | 16 |
| 6 | Watford | 9 | 5 | 1 | 3 | 14 | 14 | 0 | 16 |
| 7 | Norwich CityNorwich | 9 | 4 | 3 | 2 | 16 | 10 | 6 | 15 |
| 8 | Blackburn RoversBlackburn | 9 | 4 | 3 | 2 | 15 | 11 | 4 | 15 |
| 9 | Middlesbrough | 9 | 4 | 2 | 3 | 10 | 7 | 3 | 14 |
| 10 | Oxford UnitedOxford Utd | 9 | 3 | 3 | 3 | 13 | 11 | 2 | 12 |
| 11 | Swansea CitySwansea | 9 | 3 | 3 | 3 | 8 | 6 | 2 | 12 |
| 12 | Derby CountyDerby | 9 | 4 | 0 | 5 | 12 | 12 | 0 | 12 |
| 13 | Hull City | 9 | 3 | 3 | 3 | 12 | 13 | -1 | 12 |
| 14 | Plymouth ArgylePlymouth | 9 | 3 | 2 | 4 | 10 | 13 | -3 | 11 |
| 15 | Sheffield WednesdaySheff Weds | 9 | 3 | 2 | 4 | 11 | 15 | -4 | 11 |
| 16 | Stoke CityStoke | 9 | 3 | 1 | 5 | 10 | 12 | -2 | 10 |
| 17 | Bristol City | 8 | 2 | 4 | 2 | 9 | 13 | -4 | 10 |
| 18 | Millwall | 9 | 2 | 3 | 4 | 12 | 11 | 1 | 9 |
| 19 | Preston North EndPreston | 9 | 2 | 3 | 4 | 7 | 12 | -5 | 9 |
| 20 | Coventry CityCoventry | 9 | 2 | 2 | 5 | 10 | 13 | -3 | 8 |
| 21 | Luton TownLuton | 9 | 2 | 2 | 5 | 8 | 15 | -7 | 8 |
| 22 | Queens Park RangersQPR | 9 | 1 | 4 | 4 | 9 | 16 | -7 | 7 |
| 23 | Portsmouth | 9 | 0 | 5 | 4 | 9 | 20 | -11 | 5 |
| 24 | Cardiff CityCardiff | 8 | 1 | 1 | 6 | 3 | 17 | -14 | 4 |
| > Championship Full Table | |||||||||
