You can mix units in the watch settings, but not in Garmin Connect. People have been complaining about this for at least 10 years, but Garmin doesn't seem to care.
You can try to submit an idea here…
Trying to make Garmin notice us, I created a post for the same problem :
Custom Measurement Unit in Connect Apps - Garmin Connect Mobile Android - Mobile Apps & Web - Garmin Forums
It's kind of embarrassing…
What I'm saying is I can set the same in GCM on Android, where I can set distance to to km and weight to be lbs, for example. You need to first select the device.
Maybe consider the British newspaper headline Celsiheit scale for temperature? That's Celsius for temps near or below freezing, but Fahrenheit for summer peak temperatures? 
But seriously, I'm in the UK and I use metric for everything. I even weigh myself in kg nowadays, not stones and pounds (which many folks in Britain use: 1 st = 14 lb). I'm in my mid 50s and have known SI measures (and decimal currency) all my life. There are some hold-outs for the less self-consistent unit systems, but they tend to be older. Everything apart from road signage, beer and milk is metric units.
Do the Francophone areas use metric or Avoirdupois measures for weight?
What I'm saying is I can set the same in GCM on Android, where I can set distance to to km and weight to be lbs, for example. You need to first select the device.
And what we're saying is:
That is to set it for the device (the watch) but doesn't change how you see it in Garmin Connect app or web.
Maybe it's different for Android, but on the iOS app and the website, the settings you (Jim) are referring to only change the units on the device. It has been possible to mix units on the device for a very long time, but it isn't possible to mix units in Garmin Connect (at least the iOS app and the website.) If it's possible to mix units in Garmin Connect for Android (for display and editing in the app itself), I'm sure you'll let us know. I kinda doubt it, otherwise I'd at least expect to see this functionality on the website as well, but I guess nothing's impossible.
Maybe you could try believing multiple people when they say something works a certain way, or try understanding the nature of the problem that they are having. Or you could just assume that the various people who've been asking for this feature for 10+ years have no idea what they're talking about.
I mean people said "you can mix units on the device but not in Garmin Connect" four times before your latest reply, but you chose not to believe us.
I'm in my mid 50s and have known SI measures (and decimal currency) all my life. There are some hold-outs for the less self-consistent unit systems, but they tend to be older. Everything apart from road signage, beer and milk is metric units.
It's a Canadian thing, maybe because of our geographical and cultural proximity to the US, and maybe because Canada switched over to metric later than the UK. Nobody in Canada, regardless of age, would talk about their body weight in kg or height in cm. We also wouldn't use Fahrenheit for air temperature or talk about driving X miles per hour. For running, most ppl think in terms of km, but for strength workouts, we usually use pounds (although some weights are labelled with both lbs and kgs.)
https://www.reddit.com/r/coolguides/comments/e1hx4f/how_to_choose_measuring_units_in_canada/
And I thought we were mixed up in the UK! 
