When upgrading to Yosemite and now to El Capitan, I’ve seen the same blurry font issue occur with having an external monitor connected to a Mac Mini. Same issue has been reported with RMP and possibly other platforms.

Update 8 July 2017: I’ve encountered this same issue upgrading to macOS Sierra. The process is a little easier to follow than previously described, so I have a new post here.

I’m writing this in case someone is seeing this issue. One thing that didn’t work for me are the various posts about disabling “Use LCD font smoothing when available”, and in fact with that disabled it is worse for me. What did work is what I’ll describe below.

Rather than reinvent the wheel, I’ll point to the posts that helped me solve the problem. First, go to this post:

Force RGB in Mac OS X

Go to the top of the post, because it has been updated to include a summary of required steps, so look for “How to force RGB in Mac OS X“. Steps 1-5 there are what you need, but there is an update, “Update, 29 May 2015″, that is crucial. The folder is different for El Capitan, so follow that advice. Now, to disable the “rootless” mode temporarily, follow this post, specifically steps 1-6 that you will see in it:

Disable Rootless Mode

One gotcha that may occur is step 3, is that you may not see a Utilities menu, because it may be cut off. If so, it is there but higher than what your monitor is showing you. If that is the case, click at the top to see menu dropdown and find the Utilities menu. You may have to click around a bit to find it.

After you restart, your monitor may still be cutting off the side and top menus. If so, power down the monitor and power back up again. This fixed it for me and I recall the same thing with Yosemite.

Finish off by booting into Recovery mode again per the 2nd post, and everything should now look awesome!

I hope this helps someone else experiencing the same issue. I’m happy to help if you are seeing this and having issues getting it resolved. Feel free to send me an email.

Thanks to the original posters of the above articles, @GlennF and @imathew. Saved me a lot of time, twice!

Now that the fonts look great, I am really enjoying El Capitan 🙂