Guernsey's Scott Davidson had a debut to remember as he kicked the crucial points to help the island side beat Old Elthamians 19-18.
Tom Pool, and Davidson's conversion, gave Guernsey a 7-0 lead before three penalties from the centre saw Guernsey lead 16-3 at the break.
Tries from Renford Bennett and Patrick Smith, along with a Tom White penalty, put the visitors 18-16 ahead.
But Davidson's penalty with 20 minutes to go sealed the win for Guernsey.
The island side leapfrog Old Elthamians into seventh in the National Three London table as a result of the win.