In the contemporary northern Baltic Sea area, there are two states, Finland and Estonia, in which a western Uralic language has an official status. Several Finnic minority languages are or used to be spoken in the adjacent areas in the contact zone of Baltic, Germanic and Slavic languages. Historically, the north-eastern surroundings of the Gulf of Finland reflect a gradual spread of the language area during the past millennium and especially the late Middle Ages, while more southern areas in contemporary Estonia, and its eastern neighbourhood in Russia show longer continuity of language borders. The Finnic languages were used predominantly orally until very recently, whereas written language began to affect non-standard varieties more strongly only in the 19th and 20th centuries. The diversification of the Finnic languages in the Northeastern Baltic Sea area involves both integration and disintegration of mutually closely related varieties. This article focuses on i) the variability of changes and their correlation with emerging divergence between Finnic languages, ii) local language contacts during the late Middle Ages and the Early Modern age, and iii) the rate of linguistic change. The external powers affecting the diversity between the Finnic languages include the settling of early Swedish, German and Slavic centres among or in the vicinity of Finnic communities, the policies of medieval Catholic and Orthodox church, the extension of the trading networks of the Hanseatic League, wars and conflicts between dominant political powers in the east and west, as well as the gradual establishment of political borders. I will suggest that all these factors have played a role in the rate of both lexical and grammatical change.