The first settlers of Cortland County were primarily of English and Dutch descent and most had been born in the United States. By 1855, 10% of the residents of the city were Ireland-born. The influx of Northern Europeans, began in the 1840s, continued, but in smaller numbers, into the 20th century. As the city industrialized, large numbers of Italians began arriving as opportunities for work existed in the factories and for the railroads, at attractive pay. Small numbers of Ukrainians, Polish, Greeks and Lebanese came between 1890 and 1920. African Americans have been in the county, but in very small numbers, since the 1790s. Asian families arrived here in the late 1960s and 70s.

Between 1830 and 1860, most of the towns of the county reached their highest populations. Beginning about 1870, most rural town populations declined as farm boys were attracted to city opportunities, and some responded to the opening of far western lands. The county's overall population has increased each recent decade as there has been a notable shift from urban to rural non-farm living.