The process of sympatric speciation in nature remains a fundamental unsolved problem. Cameroon crater lake cichlid radiations were long regarded as one of the most compelling examples; however, recent work showed that their origins were more complex than a single colonization event followed by isolation. Here, we performed a detailed investigation of the speciation history of a radiation of Coptodon cichlids from Lake Ejagham, Cameroon, using whole-genome sequencing data. The existence of the Lake Ejagham Coptodon radiation is remarkable as this 0.5 km lake offers limited scope for divergence across a shallow depth gradient, disruptive selection is currently weak, and the species are sexually monochromatic. We infer that Lake Ejagham was colonized by Coptodon cichlids soon after its formation 9,000 years ago, yet speciation occurred only in the last 1,000-2,000 years. We show that secondary gene flow from a nearby riverine species has been ongoing, into ancestral as well as extant lineages, and we identify and date river-to-lake admixture blocks. One block contains a cluster of olfactory receptor genes that introgressed near the time of the first speciation event and coincides with a higher overall rate of admixture. Olfactory signalling is a key component of mate choice and species recognition in cichlids. A functional role for this introgression event is consistent with previous findings that sexual isolation appears much stronger than ecological isolation in Ejagham Coptodon. We conclude that speciation in this radiation took place in sympatry, yet may have benefited from ongoing riverine gene flow.