Brzezinski, who was last month approved by the US Senate to serve as the country’s next envoy to Warsaw, took his oath of office from Vice President Kamala Harris during a ceremony at the Eisenhower Executive Office Building in Washington on Wednesday, Poland's PAP news agency reported.
He is expected to arrive in Warsaw later this week.
Appearing before the US Senate's Foreign Relations Committee in early December, Brzezinski said he would "commit to continue and strengthen" the United States' security cooperation "with our stalwart ally Poland, where the enduring rotational presence of some 4,500 US troops defends NATO’s eastern flank."
Brzezinski also pledged that he would "work with Poland to support the government and people of a peaceful and whole Ukraine, as well as the aspirations of the Belarusian people for a democratic Belarus."
He added he would in particular "continue to underscore to Polish authorities the importance of an impartial judiciary, independent media, and respect for the human rights of all, including LGBTQI+ persons and members of other minorities."
Brzezinski previously served in the administration of Barack Obama and was US ambassador to Sweden from 2011 to 2015.
From 1999 to 2001, he served on President Bill Clinton’s National Security Council, first as a director for Russia and Eurasia, and then as a director for the Balkans, according to a statement issued by the White House in early August.
Brzezinski, who speaks Polish and French, was a Fulbright scholar in Poland and is the author of The Struggle for Constitutionalism in Poland, the White House said at the time.
"Brzezinski received a BA from Dartmouth College, a JD from the University of Virginia, and a PhD in Political Science from Oxford University," the White House statement added.
He was awarded the Royal Order of the Polar Star by the King of Sweden and is a recipient of the Commander’s Cross of the Order of Merit of the Republic of Poland, according to the White House.
Mark Brzezinski is the son of the late Polish-born Zbigniew Brzezinski, who served as US national security adviser under President Jimmy Carter and was a holder of the Order of White Eagle, Poland’s highest state honour.
Former US national security adviser Zbigniew Brzezinski, pictured in 2016. Photo: Terje Bendiksby/NTB via PAP
The previous US ambassador to Poland, Georgette Mosbacher, ended her mission after Joe Biden’s presidential election victory over Donald Trump.
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Source: IAR, PAP
Click on the audio player above for a report by Radio Poland's Michał Owczarek.