In the late 1800s, the first steel mills were constructed in Youngstown, signaling the new influence of that industry on the city's development. As a result of the city's growth, the Mahoning County seat was moved from Canfield to Youngstown in 1876. By the second half of the nineteenth century, Youngstown had become an important intersection of a number of major railway lines, including the Baltimore & Ohio, the Erie Lackawana, the New York Central, and the Pennsylvania Railroad. By 1860, the population had reached 5,300, and by 1870, 8,075 people lived in the community. With the completion of the Pennsylvania and Ohio Canal in the 1830s and the construction of railroads beginning in the 1850's, Youngstown continued to grow. Iron ore and coal deposits had been discovered near Youngstown, and an early iron industry flourished.

In that same year Daniel and James Heaton built the Hopewell Furnace. Youngstown continued to grow and was officially incorporated in 1802. Within a short period of time, ten families settled in the village along the Mahoning River. Youngstown was founded by John Young in 1797 and is within the original Western Reserve of Connecticut.Young had purchased an entire township from the Connecticut Land Company.

Youngstown is the county seat of Mahoning County, Ohio.

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