KUALA LUMPUR, Malaysia — Singapore’s new Prime Minister Lawrence Wong vowed to make “tomorrow better than today” as he took his oath of office Wednesday in a carefully planned political succession designed to ensure stability in the Asian financial hub.

A U.S.-trained economist, Wong, 51, succeeds Lee Hsien Loong, 72, who stepped down after two decades. He is the city-state’s fourth leader and the first born after Singapore’s independence in 1965.

Lee’s departure marked the end…