| Competition: | MacWhirter Welsh League Division One |
| Season: | 2012–13 |
| Winners: | West End |
| Relegated: | Tata Steel Cardiff Corinthians Barry Town |
| Biggest Home Win: | Tata Steel 0–8 Haverfordwest County 23 April 2013 |
| Biggest Away Win: | Barry Town 7–2 Goytre United 15 September 2012 |
| Highest Scoring: | Bryntirion Athletic 3–7 West End 2 February 2013 |
| Matches: | 240 |
| Total Goals: | 821 |
| Prevseason: | 2011–12 |
| Nextseason: | 2013–14 |
The 2012–13 Welsh Football League Division One began on 10 August 2012 and ended on 18 May 2013.
Aberaman Athletic changed name to Aberdare Town.
Monmouth Town, Tata Steel and Caerleon were promoted from the Welsh Football League Division Two.
Caerau (Ely), Cardiff Corinthians, Cwmaman Institute were relegated to the Welsh Football League Division Two.
Barry Town withdrew from the division and were adopted to the Welsh Football League Division Three under name Barry Town United.