Educated at Protestant universities in the Duchy of Prussia and Germany, he actively combated the Reformation implementing resolutions of the Council of Trent in Samogitia.

He became known for his devotion and work to end clerical abuses, strengthen churches and schools, and increase the number of priests.

His father Motiejus Giedraitis was a Court Marshall of Lithuania and was sent on a diplomatic mission to the Grand Duchy of Moscow in 1551.

[2] There, in 1561, he published his only known work – a 16-page Latin eulogy to commemorate the death of Katarzyna, the mother of Piotr Wiesiołowski [pl], a fellow Lithuanian student in Tübingen.

[6] The 244-line eulogy shows Giedraitis' mastery of the Latin language as well as familiarity with classical authors (Theocritus, Virgil) and Greek mythology (Philomela, Niobe, Nestor, etc.).

In February 1572, Protasewicz raised Giedraitis to members of the cathedral chapter and put him in charge of church property (custos).

They also did not want increasing Polish influence in Lithuania after the Union of Lublin – bishops were automatically granted a seat in the Senate of Poland–Lithuania.

[11] Pope Gregory XIII approved Giedraitis as bishop on 16 January 1576 and he was consecrated by Protasewicz during Easter in Vilnius.

Giedraitis conducted a canonical visitation with Jesuit Mikołaj Sędkowski (Latin: Nicolaus Sedkovius, Lithuanian: Mikalojus Sedkovskis) in 1576.

[12] Tarquinius Peculus, the plenipotentiary of the papal nuncio Giovanni Andrea Caligari, conducted an apostolic visitation in 1579.

[13] Peculus described neglected churches, poorly educated priests who could not recite the Ten Commandments or barely read Latin, did not speak local language despite living in the diocese for over thirty years, shirked their pastoral duties and held masses only occasionally, openly kept lovers and raised illegitimate children, etc.

Giedraitis established and taught at educational courses in Alsėdžiai that prepared about twenty priests; one of them – Melchior Gieysz [pl] – later became bishop of Samogitia.

He obtained two royal decrees (from Stephen Báthory on 1 August 1578 and from Sigismund III Vasa on 2 April 1592) ordering the Protestants to return lands and property usurped from the Catholic Church.

[12] Giedraitis was actively involved with the congregation – he frequently visited different parishes, delivered sermons in Lithuanian, heard confessions, taught basic catechism, etc.

Giedraitis was fond of church singing and developed a local choir and brought it to perform at Vilnius Cathedral.

[9] Sponsored by Giedraitis, he became a member of the Samogitian cathedral chapter and wrote a history of the Grand Duchy of Lithuania which was published in 1582.

[9] In the prefaces of his Catholic Postil (1599), Daukša expressed that the Lithuanian language situation had improved and thanked to Giedraitis for his works.

[20] Before the 1576 Polish–Lithuanian royal election a congress of the Grand Duchy of Lithuania's nobles was held on 20 April 1576 in Grodno which adopted an Universal, signed by Giedraitis and other participating high-ranking Lithuanian officials and nobles, which announced that if the delegates of the Grand Duchy of Lithuania will feel pressure from the Poles in the Election sejm, the Lithuanians will not be obliged by an oath of the Union of Lublin and will have the right to select a separate monarch.

[16] This ceremony consisted of Giedraitis' presentation of a luxuriously decorated sword and a hat adorned with pearls (both were sanctified by Pope Gregory XIII himself) to Báthory, while this ceremony manifested the sovereignty of the Grand Duchy of Lithuania and had the meaning of elevation of the new Grand Duke of Lithuania, this way ignoring the stipulations of the Union of Lublin.

However, Grand Chancellor Lew Sapieha refused to confirm the appointment leading to a power struggle that lasted almost a decade.

[17] The cathedral chapter tasked Jonas Kazakevičius (Jan Kosakiewicz), auxiliary bishop of Samogitia, to write a biography of Giedraitis, but it was not done.

Bishop Motiejus Valančius (1801–1875) ordered a marble tomb of Giedraitis installed in Varniai Cathedral in 1853.