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 | 15 | 9 | 4 | 2 | 25 | 11 | 14 | 31 |
| 2 | Sheffield UnitedSheff Utd | 15 | 10 | 3 | 2 | 19 | 7 | 12 | 31 |
| 3 | Leeds UnitedLeeds | 15 | 8 | 5 | 2 | 24 | 9 | 15 | 29 |
| 4 | Burnley | 15 | 7 | 6 | 2 | 18 | 6 | 12 | 27 |
| 5 | West Bromwich AlbionWest Brom | 15 | 6 | 7 | 2 | 16 | 9 | 7 | 25 |
| 6 | Watford | 15 | 8 | 1 | 6 | 23 | 22 | 1 | 25 |
| 7 | Middlesbrough | 15 | 7 | 3 | 5 | 23 | 17 | 6 | 24 |
| 8 | Millwall | 15 | 6 | 5 | 4 | 18 | 13 | 5 | 23 |
| 9 | Blackburn RoversBlackburn | 15 | 6 | 4 | 5 | 19 | 17 | 2 | 22 |
| 10 | Bristol City | 15 | 5 | 7 | 3 | 20 | 19 | 1 | 22 |
| 11 | Swansea CitySwansea | 15 | 5 | 4 | 6 | 11 | 10 | 1 | 19 |
| 12 | Derby CountyDerby | 15 | 5 | 4 | 6 | 19 | 19 | 0 | 19 |
| 13 | Stoke CityStoke | 15 | 5 | 4 | 6 | 18 | 19 | -1 | 19 |
| 14 | Norwich CityNorwich | 15 | 4 | 6 | 5 | 23 | 22 | 1 | 18 |
| 15 | Sheffield WednesdaySheff Weds | 15 | 5 | 3 | 7 | 17 | 25 | -8 | 18 |
| 16 | Oxford UnitedOxford Utd | 15 | 4 | 5 | 6 | 17 | 18 | -1 | 17 |
| 17 | Coventry CityCoventry | 15 | 4 | 4 | 7 | 20 | 21 | -1 | 16 |
| 18 | Plymouth ArgylePlymouth | 15 | 4 | 4 | 7 | 15 | 26 | -11 | 16 |
| 19 | Hull City | 15 | 3 | 6 | 6 | 16 | 20 | -4 | 15 |
| 20 | Preston North EndPreston | 15 | 3 | 6 | 6 | 15 | 23 | -8 | 15 |
| 21 | Luton TownLuton | 15 | 4 | 3 | 8 | 17 | 26 | -9 | 15 |
| 22 | Cardiff CityCardiff | 15 | 4 | 3 | 8 | 14 | 23 | -9 | 15 |
| 23 | Portsmouth | 15 | 2 | 6 | 7 | 16 | 28 | -12 | 12 |
| 24 | Queens Park RangersQPR | 15 | 1 | 7 | 7 | 12 | 25 | -13 | 10 |
| > Championship Full Table | |||||||||
