I'm going to assume you mean in America. Because in small enclaves of the globe, even today, you would find large portions of their population illiterate. Moreover, the definition of illiterate has changed over time.

When the Pilgrims arrived in Massachusetts, one core part of their beliefs was that no one could have a direct relationship with God without the ability to read the bible. So, they began the idea of universal education. Puritan communities required that all children have the opportunity to learn to read and write, so very quickly after they landed, they began to set up what would become known as the district school system. In 1642, this system was made compulsory by the Massachusetts Bay Colony. By the 1700s, schools in Massachusetts were supported through property taxes and towns were carved up into districts so that no child was walking more than 2 miles to school. And both boys and girls were sent to learn to read and write and do basic math.

By contrast outside of the Northeast, education was either a function of a church or parents paid for it out of their own pockets. So, levels of illiteracy were significantly higher in the south, even when ignoring illiteracy rates among the slaves.

The rise of the "Cult of Domesticity" and the end of slavery probably were the two factors that increased universal education. Writers like Lydia Maria Child promoted the idea that mothers were responsible for ensuring that their children would be successful in life. They also pushed for better education for daughters. Also, the end of slavery led to a influx of northern missionary teachers into the south who were there to help the ex-slaves become literate. In some ways, this created and educational arms race between white southerners and their ex-chattel. So, by the second half of the 19th century, you could say that literacy was the norm in the USA.

What we meant by literacy is not what we mean today. Writing your name, being able to read something relatively simple, and basic math would have meant a basic literacy in the mid-1800s, but we would consider that person functionally illiterate today.