[1] Angarietis was born in Obelupiai [lt] in the Suwałki Governorate of Vistula Land (present-day Lithuania) to a family of wealthy landowners.

In his memoirs, Angarietis claimed that at the age of 15 he read The Origin of the Family, Private Property and the State by Friedrich Engels which left a lasting impression on him and turned him to socialism.

[3] In Warsaw, he became acquainted with activists from the Polish Socialist Party and the General Jewish Labour Bund in Lithuania, Poland and Russia.

According to communist historian Romas Šarmaitis [lt], 13 works by Angarietis were published in the United States in 1909–1917.

[2] In the exile, Angarietis got acquainted with Elena Stasova and other communists and became involved with the Russian Social Democratic Labour Party (bolsheviks) (RSDLP(b)) joining its ranks in 1916.

After the February Revolution, he moved to Petrograd and became actively involved with the Central Bureau of Lithuanian Sections of RSDLP(b).

[3] He edited newspaper Tiesa, published numerous books, and otherwise spread the communist ideology among Lithuanian war refugees.

Many of his works were devoted to the history of the Communist Party and the revolutionary movement tracing the class conflict since the feudalism.

[3] In 1921, he wrote a 480-page manuscript on the history of CPL and accused Kapsukas of many practical and ideological mistakes that led to their failure.

A school in Šakiai, kolkhoz in his native Obelupiai, and streets in Vilnius, Panevėžys, and Kalvarija were named in his honor.