Reminders not syncing on Mac.

Thanks for your help, Peter - glad your stuff is working. It seems you are a bit offended that others are upset that their Apple products do not work as advertised. I paid the premium for Apple products because Apple products used to "just work." For what it's worth, I've logged about 10 hours on the phone with Apple Support going through every possible scenario - and they still haven't figured out the problem. Seems like our frustration is well-placed.

My reminders sync perfectly.

I held off converting the iOS13 reminders until Catalina came out (as recommended in the blurb), then hit the upgrade at the top of the page in iOS13 reminders immediately I knew I had committed to Catalina.

https://support.apple.com/en-us/HT210220

You need to sort out what is wrong with your individual setup and I tried to help.

Just means Mojave and earlier, or iOS12 and earlier, cannot read or sync them as far as I know.

https://support.apple.com/en-us/HT210220

If they can be reverted in iCloud by some kind of Mojave recovery and pushback someone will post here I expect

All of my devices show either nothing at all, or something different. Here are my 4 different devices in this order:

1. Catalina Reminders
2. iCloud.com Reminders
3. iPad Reminders
4. iPhone Reminders

Signed in under the same AppleID on all devices. Bizzaro world.

Just so there's no confusion between update to iOS13 and converting the reminders icloud data, did you hit "upgrade" within the reminders app?

Also in icloud settings is Reminders toggled on in the Mac and are you signed into your icloud account?

I found somewhat of a workaround, but it requires you use or create a Microsoft account - everything is free, though.

I installed Microsoft To Do on my Mac using my Microsoft account, and then on my iPhone I added that same account as an Outlook account under Settings | Passwords & Accounts and enabled Reminders. Now when I add reminders on Microsoft To Do from the Mac they show up in Reminders and MS To Do on my iPhone, and vice versa. You probably don't need to install To Do on your iPhone, but I just wanted to compare it with Reminders.

I realize this doesn't fix the issue with Reminders on the Mac, but I'm not able to upgrade to Catalina at this point and needed a workable solution so I went with this approach.

Thanks for the information. I've decided to migrate to the Google ecosystem and begin just using their reminders/tasks along with their Calendar. It's taking some getting used to, but after a few weeks I'm starting to really like their apps.

Bad enough Google knows what we have done without telling them what we are going to do.

Maybe not

https://discussions.apple.com/thread/250796890

These are the Apple instructions but I had to right click the list to delete

https://support.apple.com/en-gb/guide/icloud/mmc0cd7764/icloud

In the Catalina Reminders app you can delete a whole list

Right click on the list (Not on Options like the Apple article describes) and click Delete

Did you "upgrade" the idevice reminders to the new format? It was optional on iOS13