He was a delegate to the First All-Belarusian Congress and from 1918 held various positions in the Belarusian Democratic Republic, including government secretary, state controller, and head of the diplomatic mission in Berlin.

He was born in the village of Dawhinava, in the Vileysky Uyezd of the Vilna Governorate of the Russian Empire, now in the Vilyeyka District of the Minsk Region of Belarus.

[4] From 1914 to 1916, he worked at the Vilyeyka Excise Office, and from 1916 to 1917, he served as the first (responsible) secretary of the Minsk Branch of the All-Russian Union of Landowners.

[8] In July 1917, Leanard Zajac participated in municipal elections and won a seat as a deputy in the Minsk City Pride on the Belarusian People's Socialist Party list.

[2][4] On 21 February 1918, he assumed the position of secretary in the Secretariat of Belarus, established by the Rada, which served as the embryo of the future Belarusian Democratic Republic government.

[a][2][10] He continued to work in this capacity after the declaration of independence of the Belarusian Democratic Republic on 25 March 1918, in the People's Secretariat of Belarus under the leadership of Jazep Varonka.

On 12 July 1918, he became a member of the Belarusian Temporary Government, established by Jazep Varonka in opposition to Skirmunt's People's Secretariat of Belarus.

However, the condition was the recognition by the Polish side of the Belarusian Democratic Republic Council of Ministers as the official government of Belarus, which did not happen.

[5] In 1925, he participated in the Second All-Belarusian Conference in Berlin, during which he recognized the Soviet authorities in Minsk as the only center for the national and state revival of Belarus.

On 19 July 1930, he was arrested by the Joint State Political Directorate of the Byelorussian Soviet Socialist Republic and accused of involvement in the fictional anti-Soviet "Case of the Union of Liberation of Belarus".