After graduating from the Saint Petersburg Roman Catholic Theological Academy, Mironas was ordained priest in 1904 and joined the Lithuanian cultural life in Vilnius.

In late 1918, Mironas joined the Party of National Progress which merged into the Lithuanian Nationalist Union and was continuously elected to its leadership.

He had substantial influence in the new regime and handled some sensitive tasks, including attempting to mediate the conflict between Smetona and Augustinas Voldemaras.

His contemporary diplomat Petras Klimas referred to Mironas as the éminence grise (grey eminence) of Smetona.

Mironas became the successor of Juozas Tūbelis (Smetona's brother-in-law) as the Prime Minister of Lithuania after the Polish ultimatum in March 1938 and as chairman of the Lithuanian Nationalists Union in January 1939.

Mironas considered the duties of Prime Minister to be a "heavy and unbearable burden" and his government did not introduce any significant reforms.

They included Antanas Smetona, Mykolas Sleževičius, Juozas Tūbelis, Vladas Stašinskas, Petras Avižonis, Jurgis Šlapelis [lt].

[11] Mironas participated in the founding meeting of the Constitutional Catholic Party of Lithuania and Belarus organized by Bishop von der Ropp.

[12] Mironas financially supported and otherwise assisted with the publishing of Lithuanian periodicals Vilniaus žinios as well as Viltis and Vairas (edited by Smetona).

In July 1917, he was one of 19 signatories of a memorandum to the German chancellor Theobald von Bethmann Hollweg in which Lithuanians defended against the Polish territorial ambitions.

The policy of open annexation was replaced by a more subtle strategy of creating a network of formally independent states under German influence (the so-called Mitteleuropa).

[30] He also chaired ten meetings in January 1918 when council members held heated political debates regarding the Act of 11 December 1917 which called for "a firm and permanent alliance" with Germany.

[38] In late 1918, Mironas joined the Party of National Progress which merged into the Lithuanian Nationalist Union and was continuously elected to its leadership.

[39] Mironas ran as a candidate to the First Seimas in October 1922, but the party fared poorly and no members of the Nationalist Union were elected.

In September 1927, Mironas was elected to the central committee of the Nationalist Union and was tasked with editing its magazine Tautininkų balsas [lt].

[42] Despite the clear conflict between his religion and political party, Mironas did not attempt to quit either and continued to balance his loyalties.

[47][48] Due to his influence, diplomat Petras Klimas referred to Mironas as the éminence grise (grey eminence) of Smetona.

[51] In January 1938, Mironas was sent on a secret mission to negotiate with the Second Republic of Poland regarding normalizing diplomatic relations that were severed as a result of the territorial dispute over Vilnius Region.

This triggered a government crisis in Lithuania and the long-term Prime Minister Juozas Tūbelis (who was also Smetona's brother-in-law) stepped down.

[57] In his memoirs, Kazys Musteikis (newly appointed Minister of Defence) considered this government to be one of the weakest as it had no known or prominent figures.

[63] The government quickly concluded agreements relating to the cross-border railway transport and communications (mail, telegraph, and telephone).

[64] Better relations with Poland allowed to revisit the idea of the Baltic Entente and a possible military alliance between Lithuania, Latvia, and Estonia.

[65] Christian Democrats (one of the main opponents of Smetona's regime) hoped that Mironas, as a priest, would weaken the anti-religious policies of the Lithuanian government.

[66] Mironas indeed opened negotiations with the Apostolic Nuncio Antonino Arata regarding the suppressed Faculty of Theology of Vytautas Magnus University, but Smetona refused to make more than symbolic concessions.

[67] In internal affairs, Mironas supported the new Board for Public Works (Visuomeninio darbo valdyba) that was in charge of government propaganda; however, it was quickly closed due to budget cuts by the successor Prime Minister Jonas Černius.

[70] By the end of 1939, Mironas effectively retired from politics and spent increasing amounts of time at a manor he owned in Bukaučiškės II [lt] near Daugai.

[73] After the Soviet occupation of Lithuania in June 1940, Mironas was arrested by the NKVD on 12 September 1940 and accused of anti-Soviet agitation according to Article 58 (RSFSR Penal Code).

[75] NKGB managed to recruit Mironas as an informant and get him assigned to the Church of the Holy Heart of Jesus, Vilnius [pl].

[80] This time, he was accused of purposefully withholding information as well as agitating youth to join the German-sponsored Lithuanian Territorial Defense Force.

In 2007, a cenotaph was erected in Rasos Cemetery and two other signatories of the Act of Independence (Kazys Bizauskas and Pranas Dovydaitis) whose place of burial is unknown.