The share of U.S. workers born outside the country hit its highest level in decades in 2018, boosted by steady immigration and a drop in the native-born birthrate.

Foreign-born workers—those not born in the U.S. nor having U.S. citizens as parents—accounted for 17.5% of all U.S. employees in 2018, up from 17.1% in 2017, the Labor Department said Thursday. The category includes people born abroad who are now U.S. citizens, immigrants and those in the U.S. temporarily. The data also showed that foreign-born workers are increasingly...