British Prime Minister Rishi Sunak fired the chairman of his ruling Conservative Party Sunday over a "serious breach" of the ministerial code.
Truss beats George Canning, who was prime minister for 119 days in 1827.
A police report found holes in former prime minister Shinzo Abe's protection that allowed the alleged attacker to shoot him from behind.
The voting process now pits former finance minister Rishi Sunak against current foreign minister Liz Truss to replace Boris Johnson.
The victory may be propelled by what is seen as a wave of sympathy votes in a country still reeling from the shock of the former prime minister's assassination.
Ben Wallace had been tipped to be a frontrunner in the Conservative Party's leadership race — and to serve as the country's prime minister.
Johnson was booed during the queen's Platinum Jubilee — the latest sign of intense public anger over his flouting of his own government's COVID-19 rules.
Australians awoke on Sunday to a new prime minister in Anthony Albanese, the center-left Labor Party leader, though it remains unclear whether his party can form a majority government.