You can prune it if you want to, but if you're in the south, could it be suffering from drought? Or is the leaf loss just normal loss, from the base of the plant?

The leaves are falling from all over and the berries are drying up only one or two turning green I think it maybe dying .

Is it in a pot or the ground?

In the ground been there 15 years its about 6ft tall

That's odd - something's happened to it, just not sure what. Check the plant all over, particularly the base, for signs of damage, infestation, seeping or weeping or infection anywhere.

My hubby thinks it has a disease our local forest is being cut due to a disease killing the larch !! click on my profile and the mahonia is in two photo's ,it usually produce loads of grape like berries which the blackbirds cant get enough of in summer .

Phytophthera ramorum is killing larch - it does affect other plants too, certainly magnolia, camellia - Mahonia is not yet on the list, but its not impossible, specially if its nearby already. And you do live in a high rainfall region, where its pretty damp, and water droplet is one of the ways this spreads through the air. If it is that, take it out and dispose of it.

Thank you for all your advice .

I wonder, on looking at your photo, that there is a wall behind the mahonia, and if the path goes under that bed, the shrub has reached a point where the root system feels a bit confined in that pointed end of the wall, and not enough nourishment or moisture able get to the root system. Maybe take out a bit of soil and a top dressing of well rotted manure or compost or fresh soil and growmore, and more watering, might do the trick. You should get a quick response if that's what is going wrong.