Was a member of Council of the Belarusian Democratic Republic. Has published many books about Belarus, recognized the right of the people of Belarus to self-determination. Such books as "History of Belarusian (Krivichian) book", "Short history of Belarus" propagandizes the idea.
In 1922-1925 he was the prime-minister of the BDR and initiator of a dissolution of its exiled government which, in turn, did not mean a dissolution of the Council of the BDR, its parliament in exile. Lastouski was also a member of the Academy of sciences of Belarus.