Felony convictions in Baltimore dropped soon after the city’s top prosecutor took office in January 2015, and the lower rate has persisted at a time of increased violent crime, an exclusive data analysis by The Wall Street Journal found.

Under State’s Attorney Marilyn Mosby, Baltimore has posted quarterly felony conviction rates that have been lower than those under her predecessor, Gregg Bernstein, who served from 2011 through 2014, an analysis of court records shows.

About...