Activity Monitor opens at log in
I can't make it stop please help?
From the >System Preferences>Users&Groups>Login items.
unlock the padlock to make changes, use the - sign to remove from the list, if there.
When you quit an app, use the drop down menu>Quit or the key combo command Q.
Using the red, yellow, green buttons will not quit the app. (there are a few apps that will quit when using the red pill button, Activity Monitor is not one of them.)
Open items automatically when you log in - Apple Support