US President Donald Trump had just three cabinet members in place before the Haley vote: Secretary of Defense James Mattis, Homeland Security Secretary John Kelly and CIA Director Mike Pompeo, who was confirmed and sworn in late Tuesday.
The US Senate on Tuesday approved Nikki Haley, the governor of South Carolina, as US ambassador to the United Nations by a vote of 96 to 4.
The 45-year-old will assume the job at UN headquarters in New York with a clear early task: reassuring nervous governments around the world that Washington will remain engaged in international diplomacy during Trump presidency.
Trump has signalled he would like to slash US funding for United Nations climate change programmes, and he opposed a recent UN Security Council resolution critical of Israel that Barack Obama's administration had declined to veto.
Haley won the backing of Democratic Senator Ben Cardin because, he said, "she does not support efforts to slash American funding to the UN and that she would consistently vocalise US values, including universal human rights, good governance and press and religious freedom."
She also acknowledged in her hearing testimony that "Crimea is not Russian" and spoke "very strongly" about defending Ukrainian sovereignty, Cardin said, referring to the Kremlin's annexation of the region in Ukraine.
Republican Senator Lindsey Graham of South Carolina said Haley exhibited great competence as governor during trying times, notably a horrific flood and the racially charged murder of nine parishioners at a historically black church in Charleston.
"The skill set she has for bringing people together, I have seen," Graham told the chamber.
One can learn the intricacies of foreign policy; he added, "but diplomacy is something you either have or you don't."