New York City Mayor Bill de Blasio is planning to endorse Hillary Clinton for president before his planned candidates’ forum in Iowa in December, marking the end of a process that exposed Mr. de Blasio to criticism as other top Democrats lined up behind Mrs. Clinton.

The endorsement could happen long before the forum, according to a person familiar with the matter. Discussions are still under way with the Clinton camp, however, and aides for Mr. de Blasio are trying to decide how to roll out the backing.

Mr....