Sunak is the first Hindu to become UK prime minister and the second person from an ethnic minority background to enter No.10 Downing Street after Benjamin Disraeli.
Speaking at a Diwali event at the White House, Biden explained that Sunak’s win “matters”.
He began by pointing out Diwali, the festival of light “is a reminder that each of us has the power to dispel darkness and bring light to the world”.
The US president continued: “And whether it’s the United Kingdom, where just today we’ve gotten news that Rishi –”
At which point, the room erupted in applause.
Biden smiled, and continued: “Rishi Sunak is now the prime minister.
“As my brother would say, ‘Go figure.’”
After a brief pause for laughter, Biden added – with slightly raised eyebrows and a surprised gesture – “and the Conservative Party”.
As a Democrat, the US president is considered to be more on the left-wing side of politics compared to the Tories.
Still, he continued: “Expected to become the next prime minister I think tomorrow, when he goes to see the King. Pretty astounding. A groundbreaking milestone. And it matters. It matters.”
Press secretary Karine Jean-Pierre later added: “President Biden looks forward to speaking with Minister Sunak in the upcoming days and to our continued close cooperation with the United Kingdom.”
Newly elected prime ministers usually call their top allies shortly after officially being appointed by the monarch, as a diplomatic gesture of friendship.
The US president described her mini-budget as a “mistake” and said its collapse was “predictable”.
“I think that the ida of cutting taxes on the super-wealthy at a time when, anyway, I disagree with the policy.
“But that’s up to Great Britain to make that choice not me.”
He said it was “predictable” that Truss was having to abandon much of her initial plan, adding: “I wasn’t the only one that thought it was a mistake.”
Even before meeting Truss for the first time as the UK’s prime minister, Biden said he was “sick and tired of trickle-down economics” – an elusive comment some speculated could have been aimed at either the UK government or US republicans.