In discussing the impact of cotton on the American economy, we also have to discuss the impact of slavery as the one could not exist without the other in antebellum America.
The major impact of cotton was that it was one of the most important exports for the American economy. Tremendous amounts of cotton were exported to Europe where it was made into textiles. The link below tells us that cotton eventually came to represent 57% of all American exports.
Any industry that is that big supports many other industries as well. Cotton was no different. Of course, it supported people who shipped it and who insured it. But it also created a lot of economic activity through slavery. Someone had to grow the food the slaves ate. Someone had to make the clothes and the shoes the slaves wore. Someone had to insure the slaves. Most of this was done by firms in the North as the South was dedicated largely to growing the cotton.
Cotton, then, had both direct and indirect impacts. It was directly responsible for the majority of America’s exports. It was indirectly responsible for many jobs supporting the cotton industry and the slaves who worked in that industry.