How do I stop the spell checker or Autocorrect on my Mac from changing my name from "Patrick McGuire" to "Patrick Mcguire"?

Open the System Preferences. Click Keyboard, then the Text tab. Click the + button at the bottom left to add your name spelled the same way in both columns.

Hmm, guess I should have tested that. Though it's very strange you can have spaces in the second column, but not the first.

In that case, cheat. Use a combination that would otherwise normally never turn up while writing. Such as mcg, then your full name in the second column.

From then on, type mcg and it should automatically change to your full name. And then, hopefully, not try and autocorrect your last name again anyway.

Even the d**m Text Replacement lowercases the G, so what I have found that fixes that is to insert a hair space between the 'c' and the 'G'. This trick allows the proper capitalization to be used in the text replacement.

Without the hair space:

In the Text With column, place your cursor between the 'c' and the 'G'. Press control+command+spacebar to open the character viewer. In its search window, enter space and press return. Here is what you will see, and the blue center box is the selected hair space, that you will double-click to insert between the 'c' and 'G' of your name.

With the hair space:

The other benefit is that your first and last name will not split at the end of a sentence.

If when you write Patrick McGuire you get a row of red dots indicating the spelling may be incorrect, select

both words, by highlighting them. Control-Click on the highlighted text and see if you get the option to

Learn Spelling. Click on that and the mac should now accept Patrick McGuire.

By the way what days are you Patrick and what days are you Patricia, is it a weekend thing.

It's not supposed to be this difficult. You shouldn't have to resort to cumbersome workarounds either, despite the fact they ought to work too.

I'm wondering if it's a Permissions problem: Resolve issues caused by changing the permissions of items ...

Is your properly spelled name in your Contacts app?

Edit: I intended to reply to OP not you

At least it works.

Very, very bizarre that the OS doesn't autocorrect the same last name when proceeded by Patricia, Bob, David, Red or Daisy.

The only other "solution" is to turn off all three autocorrecting options as shown in my initial screen shot. The OS will still flag misspelled or unknown words with the usual squiggly red underline, but will make no attempt to alter the spelling.

Probably not. The spell check across macOS has a bit lousy of late. If I say something like this is your orange, or I can't say if you're going to be successful or not, it will sometimes flag your or you're as incorrect, even though both the spelling and grammar usage is correct. And it isn't even predictable. It may ignore the spelling (because it is correct), or may incorrectly flag it as an error.

No problem, John. Understood it was meant for the OP.

Sounds like it would be easier to just change your last name and go with the flow.

Problem solved—I am joking of course 😉 What a PITA.