The number of Americans severely behind on payments on federal student loans reached roughly 4.6 million in the third quarter, a doubling from four years ago, despite a historically long stretch of U.S. job creation and steady economic growth.

In the third quarter alone, the count of such defaulted borrowers—defined by the government as those who haven’t made a payment in at least a year—grew by nearly 274,000, according to Education Department data released Tuesday.

The...