Truss defeated former finance minister Rishi Sunak earlier on Monday, in a vote of Conservative Party members, winning by 81,326 votes to 60,399.
She takes the helm at a time when wholesale gas prices, driven higher by the Ukraine war, are hitting countries across Europe but Britain is particularly dependent on gas for electricity and heating, pushing its inflation rate above all other major economies.
During her campaign, Truss vowed to press ahead with promises of tax cuts and to deal with the growing energy crisis.
"In the north, we really need the help, I think. And she just says she's going to help all that. Like she's said she’s going to help with energy costs and living costs. And that's what we really need," said a waitress from Barnsley, Chelsea Duffy.
The announcement triggers the start of a handover from Boris Johnson, who was forced to announce his resignation in July after months of scandal saw support for his administration drain away.
He will travel to Scotland to meet Queen Elizabeth on Tuesday to officially tender his resignation. Truss will follow him and be asked to form a government by the monarch.