Yes you can flip and mirror images in Adobe Camera Raw. Here's how:

[Drop The Press]

Windows: hold the "Alt" button and click on the Rotate button in the tool bar. One is for "flip horizontal" and one is for "flip vertical".

MAC: hold the "Opt" button and click on the Rotate button in the tool bar. One is for "flip horizontal" and one is for "flip vertical".

Note: This does not work within Photoshop's built-in version of ACR, only in ACR launched from Bridge.

There are a few archived posts that say this is not possible in ACR and those answers are wrong. You can flip images in ACR.

[D Fosse]

Indeed you can. I didn't know that.

However - it does work in ACR whether hosted by Photoshop or Bridge, no difference. I suspect you're confusing with the ACR filter in Photoshop, which isn't quite the same thing. The filter is used on RGB files already open in Photoshop.

[Drop The Press]

Thanks for letting me know about Photoshop's ACR. Like you said, I was talking about the plug-in I guess (I use shortcut CNTR + SHIFT + A), so I will update the info if I can.

[John Waller]

There are a few archived posts that say this is not possible in ACR and those answers are wrong. You can flip images in ACR.

Those answers were probably right at the time.

Seems that flip and mirror were first introduced in ACR 10.2 in Feb 2018.

https://sharpenyourimage.co.uk/new-flip-option-for-mirror-images-in-dental-photography/

I cannot find any Adobe articles discussing these new features though.

[Drop The Press]

Thank you for the info! I wanted that to be known so they can possibly be updated (unable to comment on archived posts) because I was searching and only found those answers. I contacted the users who answered and one got back with me and was able to change it, hopefully we can get them both updated.

[Silentjane]

I could not get this workaround to work for me but I was able to build an action in Photoshop that I could use in bridge. Open a file in Photoshop and open the Action panel. Click the hamburger (top Left) and create an action to; select all, flip horizontal, save.
Since I had just scanned 400 positive slide film negatives upside down and needed to mirror every one, this was a lifesaver, lol. Open the folder in bridge, select the image, go to tools, photoshop, image processor, run action, (use whatever your saved action is, I named mine "Mirror"), save to same location. It created a folder within the folder of your mirrored images.