Elaine, no one can offer helpful advice without seeing some good pictures of the plant and any damage it has sustained.

Sorry rhizo_1, I would like to know how much I can prune this plant back. Or do I just need to thin it out?

Leatherleaf mahonia shrubs grow slowly and require little pruning to maintain their size. Heavy pruning in the late winter and occasional pruning in the summer, on the other hand, will improve their appearance by creating a fuller shape with denser, more attractive foliage cover.