I have a confession to make…my dining room mirror is crooked! I tried three times to get it straight, which amounted to 9 holes in the wall, before I gave up. It is good enough.
I used the same french cleat system that I had great success with for some art in the nursery. It was not quite so easy this time and it is not the cleat’s fault. The curves on the mirror make it difficult to determine if you are attaching the cleat straight. This system definitely works better on a frame or canvas with a straight edge.
In the end, the crookedness was only mildly noticeable. I opted to disguise it simply by leaning a couple monograms against one side. They distract the eye from noticing that side of the mirror is about a half an inch higher. I made the monograms from two inexpensive letters I had on hand and scrapbook paper. I made sure to also apply paper to the backside of the larger H since you can see the reflection of the back in the mirror. With this slight deception no one will probably ever know the mirror hangs a little off kilter.
I’d recommend you get one or two of those 3-M picture hangers that look like velcro and attach them to the bottom corners. They will help hold the mirror even, while the original hanger holds the weight. I love those 3-M picture hangers for smaller pictures.