The seven islands of Corfu, Paxoi, Kephalonia, Ithaca, Lefkada, Zakynthos, and Cythera (Ionian Sea, offshore western Greece) were a British protectorate with the name of “United States of the Ionian Islands” between 1815 and 1864. Although many earthquake studies have already examined the past seismicity of this area, they contain only a few data for a handful of earthquakes, for an area known to be characterized by a high level of seismicity. Against this fragmentary seismological knowledge stands a 50 yr abundant production of local documentary sources of different types and in diverse languages. For this reason and because most of the available sources had not been yet nor systematically looked into in the search for testimonies of earthquake effects, an ad hoc and comprehensive investigation was carried out. The number of records on earthquake effects is huge as well as unexpected, and the quality of the collected records is high. The 147 new macroseismic European Macroseismic Scale 1998 intensity values, accurately assigned on the basis of independent contemporary records only, are presented in the form of timelines of earthquake effects for the main towns of the four islands of Corfu, Kephalonia, Lefkada, and Zakynthos. Besides partially amending the gaps of the knowledge of these 50 yr of seismicity in the Ionian Islands, the great amount of freshly collected data suggests that historical seismological research may effectively contribute to improving the seismic scenarios of past earthquakes in many areas of the world.