His father Aaro was a carpenter and the mother, Wilhelmiina Kinnunen, a housewife who earned extra income as a seamstress for the shop of the author and social activist Minna Canth.

Haapalainen's parents wanted him to become a priest, but after graduating from the Kuopio Lyceum, he studied for two years in a business college and entered the Helsinki University Faculty of Law in 1901.

In Helsinki, he became involved with the labour movement and joined the Sawmill Workers' Union and the Social Democratic Party of Finland.

In the summer of 1906, Haapalainen represented Finnish Social Democrats in the underground Bolshevik committee planning the Sveaborg rebellion.

In April 1907, Haapalainen was elected as the first chairman of the Finnish Trade Union Federation (SAJ) which was founded in the Tampere Workers' Hall.

Haapalainen was one of the most radical persons in the labour movement, openly calling for armed revolution, and organized Red Guards in southern Finland.

As the Finnish Civil War broke out in late January 1918, Haapalainen replaced Ali Aaltonen as the Red Guards' commander-in-chief, even though he did not have any military training.

Haapalainen remained a member of the Finnish People's Delegation, but was dismissed in late April when the government had fled to Viipuri.

Later in the autumn, he fought in the Russian Civil War in a unit organized by the Finnish Red Felix Ravelin in Perm.

[2] In the summer of 1919, Haapalainen was establishing a nursing home for disabled Finnish Red Guard veterans near Kiev, but the idea never realized.

[6] In the early 1990s, Haapalainen's remains were discovered in a mass grave at a gravel pit in Petrozavodsk that was used as an execution site by the NKVD.