Arrests of people crossing the Southwestern border have plummeted 75% since May, marking one of the most dramatic drops in recent history and a sign that policy changes by the Trump administration and Mexico are reducing migration to the U.S.

U.S. Customs and Border Protection said Monday that 33,510 people were arrested after illegally crossing the border in November, marking the sixth straight monthly decline since May, when 132,000 such apprehensions marked a 13-year high.

The...