If that is a negative, then you could paint the backs of the tiles a flat black, or mask the area of glass that will not be tiled and use a flat black paint to paint the part of the mirror that will be covered with tile. I prefer to paint the glass black.

On to tiling.

You can use either 100% silicon, or a knife edge grade epoxy for your adhesive. I'm sure there are other things too, but those are tow that I'll recommend.

Here you'd mask off the area that will not be tiled to prevent adhesive from getting where you don't want it. Clean the glass with alcohol. You can squirt silicone from a caulking tube, or mix and spread your epoxy, then comb it out with a notched trowel and set your tile.

Silicone may be more forgiving in terms of open time, plus there are fewer negative consequences if things go wonky.

For clarification, a "knife-grade epoxy" is simply a high viscosity epoxy. Its higher viscosity prevents it from running everywhere. It's helpful on horizontal surfaces, and can be absolutely crucial is you are tiling a vertical surface, like a mirror already on a wall.

Best, Mongo

I found this photo online, it shows an example of how you can see the back of the tile in the mirror reflection. The thicker the mirror glass, the more of the mesh will be visible.

The color of the adhesive used to adhere the glass tile is more important the more translucent your glass tiles are. If your glass is pretty much opaque, then the adhesive color doesn't matter as much. The more translucent the glass is, the more important it becomes.

If seeing a little bit of the adhesive through the reflection in the mirror doesn't bother you, no worries. If it does, then to eliminate the reflection and without muddling the color of the glass tile, a better bet might be to paint the portion of the mirror that will be tiled black. Then adhere the tiles with a white silicon.

If your glass tiles are translucent enough to spec out using a white adhesive, then I also recommend back-buttering the tiles with white silicone before setting them.

That will ensure you get a full coverage of silicon between the tile and the glass. No entrapped air bubbles, no uncompressed ridges. Any bubbles or uncompressed ridges on the back of the tile can show through as shadowing when looking at the front of the tile.

After:

I was actually prepared to paint the back, but honestly, the reflection of the back was not a big deal since it was dark gray. So far, the tiles are still holding up.

So what is holding the mirror up? Did it have glue in addition to the clips? I am used to the clips being the only thing that holds the mirror in place.

You can also google "washer holding mirror in place" or something like that.