Hey Kevin,
Yes, if you are actually attacked by a mountain lion, fight back. There was a case here in my neck of the woods about three years ago, in which a six year old boy was attacked by a mountain lion and began dragging the boy away by the neck. The adults yelled aggressively and gave chase, and the lion let go of the child and ran off. In another case here in California about ten years ago, a lion grabbed a hold of an older man's head. The man's wife approached and screamed and grabbed a tree branch and started beating the lion, but it wouldn't let go. The man told his wife to grab a ballpoint pen from his pocket and stab it in the eye. She did, but the pen broke and the lion refused to let go. Finally, she slammed a tree branch right into its snout and it released the man. After a moment of indignation, it left the scene.
I hike in the hills around here a lot, and I've never seen a mountain lion, but I've definitely seen tracks and sign. (Photo take with lo-fi iPhone. :-) )