Rishi Sunak will become Britain’s next prime minister, prevailing in a chaotic Conservative Party leadership race on Monday after his chief rival for the position, Penny Mordaunt, withdrew. He will be Britain’s third leader in seven weeks and the first prime minister of color in its history.

The 42-year-old former chancellor of the Exchequer who is the son of Indian immigrants, Mr. Sunak won the contest to replace the ousted Liz Truss, who resigned under pressure last Thursday.