Does deleting a gmail account from one device erase all the activity for that device in the my activity section. My s.o. found my activity in our shared account. I deleted the account from my phone. Will that erase for clear the activity for that device in the account history.
No, removing a Google account from a phone doesn't affect Chrome history, Google search history, or app installation history from the time before you removed the account. You can clear all the activity from the account, but you can't only clear the activity that was from a particular phone.

It's like if you use Facebook on a friend's computer and then log out of Facebook from that computer. Logging out doesn't remove the Facebook posts you made while you were logged in. You can't automatically delete all the posts made from that one computer, because they're associated with the account, not the computer you used.