Bethlehem Steel, US Steel and coal mines, primarily. Rent the Tom Cruise movie "All the Right Moves", which was shot in Johnstown, for views of the area. Listen to the Billy Joel song "Allentown" and substitute "Johnstown" and the lyrics fit the "mentality" of the area at the time.
The "bottom" fell out in the early 80's and the area has never recovered. Considering local and state "politics", it's doubtful if it ever will. :(
But the incline is up and running again, has been for many years. :)
Thanks. It always seemed to me that some other business should have come in to the rust belt towns.