Lt-Colonel Guille took the post of Seneschal in 2000, but this role was split in 2013, as part of the island's major reforms, and he continued as the president of Chief Pleas.
This role holds several functions, with him acting as the presiding officer, returning officer and island trustee.
During his years serving the island, Lt-Col Guille has seen major changes, challenges and, in particular, reforms of the government and its election process, but he was hopeful of what the future held for the next president.
'I feel quite comfortable knowing we have a good system going in Sark,' he said.
'After years of change, I hope Sark has some stability coming up and my successor does not have quite so many difficult issues to cope with as I have had over the last decade and a half.'
Lt-Col Guille, who will stand down in October, cleared the public gallery to tell Chief Pleas of his intentions.
The new president would have to face re-election in Chief Pleas in February, for either a three- or five-year term.
On both occasions there could be contested elections if more than one person steps forward.
Lt-Col Guille said he had ended up serving longer than he intended originally, but felt now was the right time to step down.
