How do I fix a fireplace that is not centered on an angled wall?

Don't worry about the off centered ness of your fireplace as the wall itself is so short (btw center the photo of the boys over the fireplace, not the wall) Go ahead and make your mantelpiece. Is it a working fireplace? If so you may want to protect the carpeting.

You've got a real challenge! One solution might be to create a visual space to the left of the FP by installing a simple, narrow strip of molding (painted same as wall) vertically from floor to ceiling the same distance from the left side as exists on the right side. Then proceed to decorate as tho' it were a centered firebox. Alternatively, just embrace the asymmetry and decorate accordingly, not trying for formal placement. Much good luck.

Tape a piece of kraft paper the height of your desired mantle. You can play with the length to see if you like it flush to the fireplace or the end of the wall without committing. Also, if you move the tools to the other side, it will visually balance the fireplace.

To for the mantle !!!! To balance the area put a tall plant ( keep it narrow ) on the floor on the right side of the fireplace , towards the wall, coming last the edge of the wall.

Yes, I like Pamela's idea of putting something on the right. She suggested a plant, also consider some sculptural piece or a Buddha if you are into that. Or even an attractive wood holder with kindling. Maybe a big antique-looking basket or copper put (big, though) filled with kindling.

These are all great low cost ideas. But I think the problem is everything bringing attention to it being off centered. Meaning the tile, the frame around the tile and the drywall. Depending on how much this really bothers you and because I am slightly OCD, this is what I would do. Remove the tile and trim around the fireplace and retile from the corner to corner up to the ceiling. Replacing the hearth tile to match. Then add just a straight mantle, no frame. Then add accessories to the right side giving it more "weight" and balance. HTH

If you don't want to retile the whole thing right now you might be able to just paint the grout lines white so that the strange way they installed the tile won't be so apparent and the grout lines won't draw attention to the off center placement of the firebox. Then you could paint the wall the same white color as the tile all the way up to the ceiling. Take off the vertical moldings. After everything is white, another option would be to get a free standing screen and place it centered on the wall. You definitely need a mantle.

I saw on a show recently this same thing what they did was extend the tile to long end and it looked much better.

I had the same kind of thing to deal with. I found that placing things off center of the fireplace balanced it out. Now it doesn' look like the fireplace was an after thought.