It is thought that the Prūsai family were refugees from the Baltic Prussian lands, who were gradually conquered by the Teutonic Order or assimilated into the Grand Duchy of Lithuania.

After about a month Prūsas wished to become employed for the daily newspaper Birzhevyie Vedomosti[1] in which he'd earn twice the wage, so he decided to attend accountant courses, which, according to him, helped significantly in his later life.

[2] In 1903 Prūsas returned to St. Petersburg and began working in the local committee of an economic society of Moscow.

Around 1908 he also started working in the credit office of the Ministry of Finance,[1] in which he became an accountant – the highest rank for a person of non-noble origin.

However, in the works published by Russian scientists at that time, bank managers, the most active members of the resistance, were arrested by soldiers at night and taken to Smolny for "interrogation" by Felix Dzerzhinsky.

[6] In 1918, along with other Lithuanian intellectuals such as Kipras Petrauskas, Stasys Šilingas and Justinas Zubrickas [lt], Prūsas reached Vilnius after a month-long journey, living mainly by exchanging older money into local currency.

When their monetary supply began to run out, Prūsas along with Juozas Strazdas established a colonial goods store.

In a meeting of representatives of credit companies held in Kaunas, then the capital of Lithuania, he proposed to liquidate all companies of this type that operated in Lithuania, and instead prepare new laws letting small credit societies, commercial and lank banks to develop;[6] the government did not adopt this proposal.

Prūsas retained his advisory position for one term, noting in his memoirs that as a member of the Lithuanian Nationalist Union, he will not be elected for a second time.

In addition to his banking work, Prūsas participated in the establishment of various joint stock companies like Lietuvos Loida and Nemunas,[7][1] and was elected to their boards.