Between 2007 and 2009 he worked as an assistant to the mayor of Vilnius and, later, to members of the European Parliament Juozas Imbrasas and Rolandas Paksas.

A member of the Order and Justice party, Žemaitaitis was elected to the Tenth Seimas in 2009, in the by-election in the single-seat constituency of Šilalė–Šilutė.

[15] He was elected as MP representing the single-seat constituency of Southern Samogitia,[16] a new name for the Šilalė district, now defined to exclude Šilutė.

In October 2019, after Žemaitaitis backed an effort to strip Viktoras Pranckietis of his position as Speaker of the Seimas, the leader of the Lithuanian Farmers and Greens Union party Ramūnas Karbauskis urged Žemaitaitis to resign from his Deputy Speaker position, calling the move a "betrayal" of the ruling coalition.

[10][25] In September 2018, while giving an interview to a Russian-language weekly newspaper Ekspress nedelya, Žemaitaitis stated that he was against what he referred to as "anti-Russian sanctions" and advocated for talks.

[29] The first two posts on 8 May criticised the state of Israel for human rights violations against Palestinians, and in particular the destruction of an EU-funded school in the West Bank.

[33] During his remarks in Parliament, Žemaitaitis asserted that his statements were targeted towards Israel's actions against Palestinians, before the start of the 2023 war,[34] and afterwards clarified that he would support a death sentence for Benjamin Netanyahu.

[38] Following the 2024 Lithuanian parliamentary election, the decision of the victorious Social Democrats to invite Dawn of Nemunas into the new ruling coalition provoked local and international backlash focusing on Žemaitaitis' past statements.

Politicians in the United States, Germany, Poland, and Israel criticised the decision to include Dawn of Nemunas, as did civil society groups and protesters in Lithuania.

[42] On 19 November 2024, the Prosecutor's Office began investigation into alleged defamation of Ingrida Šimonytė made by the politician on Facebook.

[43] In November 2024, the Lithuanian Police had begun investigation of another Facebook post made by Žemaitaitis, on grounds of BK 170.2 pertaining to ethnic hatred.

[44] In the post he called for protestors to gather with candles in front of the home of Vytautas Landsbergis, Lithuania's first post-Soviet leader and the former head of the opposing Homeland Union party.

This post was made as a response to a protest that was planned to be held in Independence Square in Vilnius against the inclusion of Dawn of Nemunas in the ruling Seimas coalition.