Venclauskis joined the Social Democratic Party of Lithuania in 1902 and was active during the Russian Revolution of 1905.

His lucrative private attorney practice, which focused on commercial representation and arbitration, allowed him to become a philanthropist.

Together with his wife, he sheltered and raised over a hundred orphans and foundlings and otherwise supported various students.

They organized Jėga Society (power, strength), chaired by Venclauskis, to support the school which opened in April 1918 with 200 students.

He served until 17 April 1919 during the chaotic post-war period when the city changed hands during the Lithuanian–Soviet War.

[8] After serving in the national parliament, Venclauskis returned to the Šiauliai City Council as chairman in 1925–1931 (Jackus Sondeckis [lt] was burgermeister at that time) and as member in 1935–1939.

[8] Venclauskis, as a member of the Social Democratic Party of Lithuania (LSDP), was elected to the Constituent Assembly in May 1920 and reelected to the First Seimas in October 1922.

At the Constituent Assembly, he was a member of various parliamentary commissions, including those on economy and draft constitution.

[2] Due to the emergency caused by the Żeligowski's Mutiny in October 1920, the assembly established the seven-member Small Seimas to continue legislative function and temporarily adjourned.

[9] When drafting the new constitution, Venclauskis as other social democrats opposed creating the position of the President of Lithuania.

During discussions of one of these interpellations (in which LSDP asked why the police was not allowing or disrupting the celebrations of the International Workers' Day in Lithuania), Venclauskis engaged in a heated argument with LKDP leader Mykolas Krupavičius and was removed from the meeting.

Since LKDP ignored most of the proposals put forth by LSDP, Venclauskis and other members of his party protested by not voting for some of the major laws passed by Constituent Assembly, including the constitution and the land reform.

When the Seimas was dissolved in March 1923, Vencklauskis returned to his private law practice and the Šiauliai City Council.

[3] Venclauskis was mainly a commercial attorney, representing Swedish and Swiss firms (including defense contractor Oerlikon Contraves).

He worked on some high-profile cases, including a murder of a child in Žagarė, trials of several participants in the Tauragė Revolt, and corruption of diplomat Vaclovas Sidzikauskas.

[3] The case earned him enough fees to finance the construction of a palatial house by architect Kārlis Reisons [lt] in 1925–1927.

[1] A fine example of modern interwar architecture, the house now hosts a section of the Šiauliai Aušros Museum [lt] and is listed on the Lithuanian Real Heritage Register.

[10] In August 1919, Venclauskis co-founded and became board member of the Šiauliai Union of Consumer Cooperatives.

[11] Together with Vladas Sirutavičius [lt], Venclauskis founded construction corporation Pastogė (shelter).

It had plans of building a large culture center and a hotel, but managed to finish just a movie theater before the outbreak of World War II.

As a compensation, the Tsarist government granted a privilege that three future generations of Jakševičius could attend the school for free.

[13] Venclauskis did not spend much time with the children and was distant with rare exceptions on vacations at the seaside or holidays, but gladly provided financial support.