This paper presents the results of recent archaeological work conducted at the ruined site of Kisimani Mafia, Mafia archipelago, Tanzania. The site is one of the two stone town sites on the archipelago, the other being Kua. Despite being fairly well reported in scholarly accounts, few archaeological investigations have been carried out, with Neville Chittick’s excavation at the site in the late 1950s being the only one. As the then prevailing scholarly interpretation of all ruined sites on the East African coast, the founding of Kisimani Mafia and Kua was attributed to the Persians and Arabs, respectively. It was suggested that Kisimani Mafia dated to the early second millennium CE, and was then regarded as the earliest settlement on the archipelago. This endeavour sought to review the cultural history of the stone town settlement in relation to the remote settlement history of the archipelago and the coast generally, currently known to date to the early first millennium CE, or even before that. This study involved an archaeological survey and excavations carried out on an open-air site in 2018. An array of cultural materials was recovered, including pottery, beads, glass, coins, slag and metal objects, as well as bone and shell remains. This paper, however, is based on the results of pottery and chronometric analyses, the results of which firmly established that the stone town site was occupied from a much earlier period than was thought. The cultural history spans the Triangular Incised Ware/Tana Tradition period through the Plain Ware period, to the end of the Swahili Ware period. The first two periods, which evolved from the Early Iron Working culture of the archipelago, were when the Swahili stone town settlement was founded.