| Competition: | MacWhirter Welsh League Division One |
| Season: | 2009–10 |
| Winners: | Goytre United |
| Relegated: | Ton Pentre, Ely Rangers, Bettws, Dinas Powys, Caerleon |
| Biggest Home Win: | Afan Lido 9–1 Dinas Powys 24 March 2010 |
| Biggest Away Win: | Caerleon 0–6 Goytre United 29 August 2009 |
| Highest Scoring: | Afan Lido 9–1 Dinas Powys 24 March 2010 |
| Total Goals: | 1066 |
| Average Goals: | 3.48 |
| Prevseason: | 2008–09 |
| Nextseason: | 2010–11 |
The 2009–10 Welsh Football League Division One began on 15 August 2009 and ended on 22 May 2010. Goytre United won the league by one point.
West End, Ely Rangers and Garden Village were promoted from the Welsh Football League Division Two.
Cwmbran Town, Croesyceiliog and Newport YMCA were relegated to the Welsh Football League Division Two.