Involvement in Lithuanian language-related activities by two generations of the Mielcke (Lith. Milkus) family, who had been active in the Lithuanian cultural arena of the Kingdom of Prussia over a period of eight decades in the 18th century, started with Peter Gottlieb Mielcke (Lith. Petras Gotlybas Milkus, 25/10/1695, Tilsit [Lith. Tilžė] – 09/06/1753, Mehlkehmen [Lith. Mielkiemis]), a teacher at the Lithuanian Language Seminar at the University of Königsberg (1723– 1725) who later served as an evangelical Lutheran priest in Prussian Lithuania, i.e. in Georgenburg (Lith. Jurbarkas, 1726–1735) and Mehlkehmen (1736–1753). The purpose of this article is to restore the missing links of Peter Gottlieb Mielcke’s family and its four generations, paying attention to genealogical relations with well-known contemporaries, especially those who contributed to Lithuanian culture. The genealogical reconstruction of four generations of the Mielcke family was based on primary authentic sources: the surviving church books of Prussian Lithuania, which are currently stored in the Evangelical Central Archives (Ger. Evangelisches Zentralarchiv) in Berlin and in the State Archives in Olsztyn (Pol. Archiwum Państwowe w Olsztynie), Poland. Microfilm copies from church books that had disappeared during World War II , which had been made before the war from the originals and are now kept at the Saxon State Archives in Leipzig (Ger. Sächsisches Staatsarchiv, Staatsarchiv Leipzig), were also used. In addition to the church books, the author referred to various archival documents stored in the Secret State Archives Prussian Cultural Heritage Foundation (Ger. Geheimes Staatsarchiv Preußischer Kulturbesitz) in Berlin. In restoring the subsequent generations of Peter Gottlieb Mielcke’s descendants and attempting to find any traces of Lithuanian language-related activity, secondary sources were also used. The most important of these are the biographies of East and West Prussian priests written by Friedwald Moeller (1894–1964) and published in the book entitled Das „Zwischenmanuskript“ zum Altpreußischen evangelischen Pfarrerbuch (vol. 1–7, Hamburg, 2012–2014). The conducted study led to the following conclusions: 1. Peter Gottlieb Mielcke was born in the family of Maria (?–after 1695) and Peter (?–after 1695) Mielcke, who were residents of Tilsit. His parents considered themselves Germans: their eight children were given German names, all of them were baptised in the German Church (Ger. Deutsche Kirche) in Tilsit, and his father’s surname was recorded in the church’s baptism register in its German form: Mielcke, Mielke. The maternal genealogical roots of Peter Gottlieb Mielcke in Prussian Lithuania, Tilsit, originate from the Engelbrecht family and date back to the 16th century.