The police forced him to flee to Switzerland where he received engineering education and firefighting training.

[1] He studied at the Real Gymnasium in Vilnius but was expelled for anti-Tsarist activities after he joined the Polish Socialist Party.

[1] There he joined workers' organization Borba (fight, struggle), was arrested by the police, and exiled to Switzerland.

In 1903, he joined Polish socialists in London and helped them publish various revolutionary brochures in Belarusian.

In January 1919, as a representative of the Belarusian minority, Semashko was sent by Lithuania to the Paris Peace Conference.

[3] His role at the conference became more prominent after a Belarusian delegation, led by Prime Minister Anton Luckievich, arrived to Paris in summer 1919.

In mid-June 1919, Semashko issued a memorandum that unconditionally recognized the territories of the former Vilna and Grodno Governorates as Lithuanian.

According to Tomasz Błaszczak, Semashko began to take an anti-Belarusian stance and came to fully support Lithuania's independence.

[3] After the elections to the Constituent Assembly of Lithuania in May 1920, Jazep Varonka was replaced by more pro-Lithuanian Semashko as the Minister for Belarusian Affairs without portfolio.

[5] He established and briefly edited monthly journal Lietuvos gaisrininkas (Lithuanian Fireman) in 1923[6] and published a book on firefighting techniques in 1926.