I had not found the first link you offered, which seems right on target. I did find a lightdm.conf file, that’s the one I said had just one entry and it was not relevant. Now I have some pointers to possible settings I can try.
I’m not near the PC right now, but it’s clearly MINT 18 and was supposedly the latest, so presumably 18.3.
This system didn’t have an mdm.conf file, in fact “find” didn’t find anything on the disk about “mdm”. Online I saw references to it, but didn’t find the setting that I may need, "AutomaticLoginEnable=False”, which I’ll keep in mind as a backup.
Stephen: You mentioned a live DVD, thanks for the nudge. Since I am now supporting two Mint systems besides my own desktop, I think it’s time to learn how to make a USB backup boot. We do have the “tom” account doing the usual sudo and admin things, but I’ve been afraid to wipe out the “user” account because the auto-login might hang up looking for it. I’d thought about a live DVD as insurance, but your mention has pushed me into getting it done.