So I made my eyebrows on one side using particles and want to mirror it on the other side. I added the Mirror modifier and made sure that it is the first stack. I also turned on “Use Modifier Stack”, however, it just made my eyebrows disappear. Does anyone here have experience mirroring hair particles??
It’s quite a mess, using generative modifiers and hair particles together. I also got bitten quite a few times with hair disappearing.
The issue is, when you have a hair particle system on a mesh, you may not do anything with it that changes the number of vertices, topology and such. This applies to both generative modifiers and working in edit mode. If you do, your hair will be all over the place or gone completely.
For this reason, IMHO it is better to not put the particles directly on the render mesh, but use a simpler proxy mesh which doesn’t need any modifications later.
As to the concrete question, if you want to put hair on the mirrored side of a mesh with an unapplied mirror modifier, you have to enable ‘use modifier stack’. But the hair will not get mirrored automatically (can you have different ones on both sides). What you can do to actually mirror them, is to create one side, select all hair in particle edit and do Particle->Mirror.
I went to Particle Edit, went to Select>All, then Particle>Mirror. Nothing happens. 
Just to be sure, you enabled ‘Use Modifier Stack’ for the particle system and placed to mirror modifier before the particle system on the modifier stack?
If that isn’t the reason, then I don’t know, without seeing the blend file.
Yes I did all that. Watch me do it here: https://youtu.be/uJ7Tv5mZQnM
And here is the Blend file: HairBaldGuy1.blend (3.2 MB)
Ah, I see. Due to the issue I talked about in my first post, it probably is too late to rescue the brow you already have. What you can try is to do ‘Disconnect Hair’ then set up the modifier as mentioned, then do ‘Connect Hair’. It may or may not work.
Beside that, if you create new hair, the mirror operation should work. When I take your blend file, add some hair, select+mirror works for me as expected.
I redid the hair, still didn’t work. Can you show me a recording of exactly how you are doing it with my blend file?
Nothing special. Select add tool. Left click to add particles. Select all. Mirror.
Video cannot be played 
Looks like the video works in my desktop, but not on my cellphone! Thanks! Looks like I am able to do it. 