I guess it depends what the surface finish on the mahogany is. There's two problems - having a finish that new paint will stick to, and covering over a very deep colour (especially if you wanted a pastel shade). To satisfy the first one, the surface needs to be clean, dry and free from flaking under-surface. The tradtional way of achieving this is sandpaper but if the fireplace has lots of nice carvings / shapes its a lot of work. What about paint stripper, painted on then washed off and dried?
The second problem can be solved by using an aluminium primer paint on the dark wood, which then more easily covers with white odr other pastel shade.