Immigrants were vital to the growth of the US before the Civil War. New immigrants from Ireland and Germany made up the bulk of the labor pools that worked on creating railroads, canals, and factories. This nearly limitless supply of cheap labor allowed the US to build a strong industrial base prior to the Civil War. Many immigrants also went West in search of a better life.

Immigrants were drawn to the United States based on the promise of living a better life than they did in the Old World—when this life was unattainable in the cities, many immigrants went west in search of cheap land and better opportunities. They brought their music, food, and traditions with them—over time, many of these formerly ethnic holidays, such as St. Patrick's Day, have become popular in mainstream American culture.

Indirectly, immigrants helped to fuel the rise of American cities. In order to live closer to their factory jobs, many immigrants decided to live in their own ethnic neighborhoods in major cities such as New York and Chicago. These immigrants printed their own native-language newspapers that lasted into the twentieth century—many of these were shut down after World War I.

Immigrants also supplied many of the votes that supported political bosses since the bosses often tried to appear as generous patrons to poor immigrant families in order to buy their votes later. Immigration also fueled the US's first political party based on xenophobia, the Know-Nothing Party. Native-born white Americans worried that the immigrants would take all the jobs and contaminate American Protestant values with Catholicism brought over from Germany and Ireland. In this respect, immigration influenced American society by revealing some aspects of American culture as unwelcoming.