As a student at Mitau Gymnasium, Šliūpas read works by John William Draper that he later credited for laying the foundations for his lifelong dedication to freethought, promotion of science, and criticism of the Catholic Church.

He organized Lithuanian miners in response to the Lattimer massacre in September 1897 and during the Coal strike of 1902 and unsuccessfully ran in the elections to the United States House of Representatives in 1896 and 1900.

There, Šliūpas studied Russian and Polish languages and witnessed moral degradation of the clergy, even his uncle who lived with his housekeeper in sin and somehow managed to obtain large sums of money.

Together with Janavičius, Šliūpas spent summer 1881 visiting various locations across Lithuania collecting funds to support members of revolutionary groups Narodnaya Volya and Black Repartition suffering from political repressions.

[16][17] In Moscow, he visited the Rumyantsev Museum, attended meetings of the Russian Geographical Society, listened to debates between professors Dmitry Ilovaysky and Fedor Korsh [ru] on the origins of Slavs.

[19] In summer 1882, Šliūpas visited East Prussia to investigate the possibility of publishing a Lithuanian-language periodical – the printing of Lithuanian texts in the Latin alphabet was banned in the Russian Empire since 1864.

[30] Searching for means of living, he rejected an invitation to join Narodnaya Volya and was about to emigrate to Chile to become a farmer, but received an offer from Martynas Jankus to become the editor of Aušra, the first Lithuanian-language newspaper published in Ragnit (now Neman) in East Prussia.

[a 1][36] He introduced some socialist themes to Aušra – articles promoting economic development with the ultimate goal of social equality, descriptions of poor conditions of peasants and manor workers, etc.

[37] When the Polish newspaper Dziennik Poznański criticized Aušra for its anti-Polish attitudes and separatism, Šliūpas engaged in a debate with its editors regarding the cultural and historical relationship between Poland and Lithuania.

[40] Šliūpas visited Kaunas where he learned that Pyotr Shchebalsky, editor of the Russian edition of Dziennik Warszawski [ru], raised the issue of the Lithuanian press in Congress Poland which included Lithuanian-inhabited Suvalkija.

It was a small four-page publication that mainly printed Šliūpas' texts that focused on Lithuania's revival, promoted a union with Latvia, discussed children's education, advanced freethinking and socialist ideas.

[54] This elicited protests from Polish groups as well as from within the Mutual Aid Society of Saint Casimir and, on 27 April, Tvarauskas fired Šliūpas leaving him with no money or a place to stay.

[59] Despite the difficulties, Šliūpas continued to organize a separate Lithuanian parish and invited priest Antanas Varnagiris from Independence, Wisconsin, who arrived in June 1885.

[65] In 1887, faced with constant criticism and attacks, Šliūpas published a Polish-language brochure Litwini i Polacy (Lithuanians and Poles) to explain his ideology on the Polish–Lithuanian question.

[67] He also translated and published Patkulis, a historical-political drama by the German writer Karl Gutzkow about Johann Patkul, a Livonian nobleman and an instigator of the Great Northern War (1700–1721), who was portrayed as a Latvian fighting for his nation's freedom.

Hoping to increase the readership, Šliūpas moved from New York to Shenandoah, Pennsylvania, where many Lithuanian immigrants worked in local coal mines, in early 1888.

[92] On 28 January 1894, in response to the Kražiai massacre in Lithuania, Šliūpas and Dembskis organized a protest in Shenandoah during which they publicly denounced not only the Russian Tsar but also the pope.

[119] Lithuanians wanted to organize a protest march in support of the Russian Revolution, but Šliūpas proposed a multipartisan congress (perhaps inspired by the Great Seimas of Vilnius).

They gathered separately in Brooklyn and decided to establish their own organization, the Lithuanian National League of America (Amerikos lietuvių tautinė sandara), as well as two foundations (chaired by Šliūpas) to provide financial aid to war refugees in Lithuania.

During that time he published two political booklets, Lithuanian Lietuvių-latvių respublika ir Šiaurės Tautų Sąjunga (Lithuanian–Latvian Republic and the Union of Nordic Nations) and English Essay on the Past, Present, and Future of Lithuania.

[145] Šliūpas returned to United States in May 1918 and became vice-chairman of the Lithuanian Executive Committee organized by a joint Catholic and nationalist conference in New York in March 1918 and chaired by Tomas Naruševičius [lt].

[148] When Henry Cabot Lodge, the new Senate Majority Leader, delivered a speech in which he urged to preserve the territorial integrity of Russia, Šliūpas wrote a memorandum demanding independence for Lithuania and managed to get it published in the Congressional Record on 29 August 1918.

[149] Šliūpas also contacted Robert Lansing, the Secretary of State, Frank A. Golder, a member of The Inquiry preparing for a post-war peace conference, Theodore Roosevelt, the former president.

He delivered speeches on Lithuania's independence to the Union of Democratic Control as well as Lithuanian immigrant communities in London, Glasgow, Burnbank, Edinburgh, Liverpool, Manchester.

[162] Šliūpas returned to United States to organize his affairs for the permanent move to Lithuania and at the same time sold about $50,000 worth of shares of the company to Lithuanian Americans.

[182] He worked on lowering electricity prices, ensuring street cleanliness, improving transportation (e.g. asking city residents not to use sidewalks to ride bikes or herd cattle), etc.

[196] On 23 November 1935, during a military celebration at the Vytautas the Great War Museum which was broadcast live over the radio, Šliūpas delivered a critical speech with the president and other politicians of the Lithuanian Nationalist Union in attendance.

[193][197] In April 1939, he wrote a letter to the government of Prime Minister Jonas Černius, which he managed to get published only in Naujoji gadynė in United States in September.

[204] In United States, Šliūpas delivered speeches and lectures in twenty different communities of Lithuanian Americans, including Pittsburgh, Detroit, Grand Rapids, Chicago.

[217] He communicated with Petras Kubiliūnas, the General Counsel of the Generalbezirk Litauen, and even floated an idea of a special commission to visit Adolf Hitler to present a proposal for Lithuania's independence.