Guillaume Hareby was the step-brother of Sir Richard Harliston, who was appointed Jersey's first Governor (or Captain, as the role was known then) in 1471 after he had ended the French occupation of the island in 1467. Hareby is sometimes described as joint-Captain. An Act of 13 January 1478 confirming Harliston's appointment, shows Hareby as his "assistant or associate", with the same powers and responsibilities.
Whatever his initial responsibilities, it seems that Hareby assumed the responsibilities of Bailiff for two periods, although why he was not in office in 1482 and 1483 is not known. He appointed Guillaume de Hamptonne as Lieut-Bailiff from 1480 to 1481, so his responsibilities appear to have ended with the termination of Hareby's first period of office. Jean Poingdestre is shown as being appointed Lieut-Bailiff in 1485, and being in office in 1486 and 1492. This would appear to be the same Jean Poingdestre who was Bailiff from 1467 to 1476. It is probable that Hareby fell from grace alongside Harliston when fluctuating fortunes of the two sides in the War of the Roses led to the latter being besieged in the same Castle that he had rescued from the French 18 years earlier. Poingdestre was almost certainly brought back in the role of Lieut-Bailiff before the appointment of the next Bailiff, Clement Le Hardy.
| Predecessor | Successor | |
| Jean Poingdestre 1468-1477 |
Guillaume Hareby 1479-1481, 1484-1485 |
Clement Le Hardy 1486-1493 |
