They took the lead with a penalty midway during the first half and extended their lead soon after that.
Mawnan managed to get a goal back and the score was Mawnan 1 St Buryan 2 at half-time.
The second half was a more edgy affair and St Buryan held on to win. The game was played at Gala Parc, Porthleven.
St Buryan will play Looe Town who overcame Lanreath in the replay of the Eastern Divisional final.