Port cities across the world were deeply affected by the First World War. They had to adapt quickly to the influx of soldiers and labourers, as well as to resulting population changes that challenged and strained local structures. The pressure of these changes was especially felt in colonial port cities, as new arrivals – from abroad and from the hinterland – had the potential to undermine colonial order and control. In the period of 1914-1918 colonial authorities around the Indian Ocean saw local concerns regarding order and control overridden by global developments and imperial demands. Mombasa, located on the East African coast, provides a potent example of this situation. In the war years, Mombasa turned into a military hub, with thousands of troops passing through it to serve in the East African campaign. These external arrivals were mirrored by the movement of people from the hinterland into the city seeking to fill positions in the local war economy. In this dynamic context, racial borders, social conventions, and economic patterns were continually challenged and transgressed. This article explores these developments and reactions they triggered among Indians, Africans, and Europeans in Mombasa at the time of the global war.