This is a shared resource for Columbia County and Montour County. Columbia County was created from Northumberland County in 1813. Montour County was created from Columbia County in 1850. Since these tax lists are from the late 1700's, they are from the time when, what we know as Columbia and Montour Counties, was a part of Northumberland County. On April 9, 1772, three townships were created that would cover this area. They were Wyoming Twp., Turbott Twp., and Augusta Twp. Mahoning Twp. was created in Feb 1775. Check the tax lists for these four townships out, maybe you'll find someone you are looking for!
PAGenWeb 
PA USGenWeb Archives 
USGenWeb