He was born in the village of Vickaŭščyna close to Minsk. His birthplace later became a subject of one of his works.
In 1929 Mikałaj Ułaščyk graduated from the Belarusian State University, specialised in history. In 1930 he was arrested by the NKVD together with a large number of Belarusian intellectuals accused of "anti-Soviet activity" and "nationalism" within the framed-up "Trial of the Belarusian Liberation Union" (Союз освобождения Беларуси). He was deported to a labor camp close to Vyatka. After his release he was again twice arrested and imprisoned in the following years because of his active opposing the official Soviet propagandist historiography.
Mikałaj Ułaščyk became Doctor of sciences and author of numerous works on the History of Belarus and the Grand Duchy of Lithuania related to the research of chronicles, source documents and archaeography.