Truss conceded last week that she could not deliver on her plans - but only after her attempts triggered market chaos and worsened inflation at a time when millions of Britons were already struggling with soaring borrowing costs and rising energy and food prices.
The party is now desperate for someone to right the ship after months of chaos under Truss and Johnson, who quit in July after becoming mired in ethics scandals.
Sunak was chosen as Conservative leader after becoming the only candidate to clear the hurdle of 100 nominations from fellow lawmakers to run in the party election.
Sunak defeated rival Penny Mordaunt, who may get a job in his government, and the ousted Johnson, who dashed back from a Caribbean vacation to rally support for a comeback bid but failed to get enough backing to run.
As well as stabilizing the UK economy, Sunak must try to unite a governing party that has descended into acrimony as its poll ratings have plunged.
Conservative lawmaker Victoria Atkins, a Sunak ally, said the party would "settle down" under Sunak.
"We all understand that we've now really got to get behind Rishi - and, in fairness, that's exactly what the party has done," she told radio station LBC.