From 1763 until his death on 22 October 1778 Mackellar was engaged in rebuilding and fortifying the harbour at Mahon. One of the main reasons for the relatively rapid fall of Fort St Philip in 1756 had been the proximity of Philip's Town to the fortifications. This settlement had sprung up around the fortress in response to the lack of quartering for soldiers within the fort itself and in the inevitable way that taverns, sutlers, families and less reputable women coalesced around the armies of that time. When the French began their siege operations they were able to use the town to conceal their approach from the fire of the fort and to provide ready material for the building of their batteries.