Bill de Blasio took office Wednesday as New York City's 109th mayor, returning City Hall to Democratic control for the first time in 20 years and ushering in a progressive agenda aimed at narrowing the gap between the wealthy and poor.

Mr. de Blasio, 52 years old, succeeded Michael Bloomberg, the billionaire businessman who stepped down Tuesday night after 12 years as chief executive of the nation's largest city. Mr. de Blasio's landslide victory in November ended a streak of losses for Democrats that began when former Mayor...