The shifter on my old mt. bike will not shift the front gears. I don't feel any clicking in the shifter, but it does move the cable when I try to shift from the the small front sprocket to the larger ones. Could this just be slack in the cable, or do i need a new shifter?

yes there is slack in your cable . you might need new cable and housing if that doesn't work then you need a new shifter .

You might describe the type of shifter you have.

Does the FDER move toward the larger ring and then move back when you let off the "lever"?
Sometimes, flushing with a solvent, like WD-40, will thin out the hardened grease and allow the pawls to engage (again).

Its an old Shimano Deore XL shifter for a seven speed drive train, it does move the fd toward the big chain ring and then goes back when let off the shifter. I will try the wd40 and see if that works. Thanks for your info.

If just shooting it with WD40 doesn't do it, I've had luck opening shifters up, spraying them good and physically moving the pawls back and forth till they move freely.