How can I rotate videos taken with my Canon 5D Mk3 so that I can view them the correct way in Lightroom - I am using Lightroom CC 2017 and Mac with OS Sierra
It does not seem that Lightroom supports rotating a video. But you can easily rotate in iMovie.
Also quick time
How to Rotate or Flip Videos in Mac OS X
This method seems to do more then just rotate the video. A .mov file I tried it on went from 101 MB to 22 MB after just rotating and saving using QuickTime.