Specifically, when did they start to industrialize? I read somewhere that the reason that they moved away from slave labor was because of the high overhead and the short crop season. Was it more of an agrarian economy post Revolutionary War? Or did it already start showing signs of textile manufacturing?
Well, a great deal of North's economy was based on the sea. Shipping, whaling, and trading were three of the pillars of the economy. Whaling has been largely forgotten, but was a big international business in that era.
Industrialization followed the war pretty quickly. By 1800, there were already quite a few factories. In general, towns concentrated on producing certain goods. Danbury CT, by 1800, was the largest producer of hats in America. Waterbury was known for its clocks, etc. By 1800, there were also a number of textile mills.
While there were many farms, the soil is very rocky and not very productive. The area has also had a ready supply of agricultural produce from the west as well. The Erie Canal could easily bring in food from the west to feed growing towns out east.