In this study, we measure the contribution of immigrants and their descendents to the growth and industrial transformation of the American workforce in the age of mass immigration from 1880 to 1920. The size and selectivity of the immigrant community, as well as their disproportionate residence in large cities, meant they were the mainstay of the American industrial workforce. Immigrants and their children comprised over half of manufacturing workers in 1920, and if the third generation (the grandchildren of immigrants) are included, then more than two-thirds of workers in the manufacturing sector were of recent immigrant stock. Although higher wages and better working conditions might have encouraged more long-resident native-born workers to the industrial economy, the scale and pace of the American industrial revolution might well have slowed. The closing of the door to mass immigration in the 1920s did lead to increased recruitment of native born workers, particularly from the South, to northern industrial cities in the middle decades of the 20th century.