How do I actually disable mouse acceleration in Linux Mint?

If you're using an X11 session and the libinput driver, you can set the flat acceleration profile in an Xorg configuration file.

Create the directory /etc/X11/xorg.conf.d if it doesn't exist, then create /etc/X11/xorg.conf.d/50-mouse-acceleration.conf with administrator privileges. Put these lines in the file, each on its own line:

Section "InputClass"Identifier "My Mouse"Driver "libinput"MatchIsPointer "yes"Option "AccelerationProfile" "flat"Option "AccelSpeed" "0"EndSection

Save the file and restart your X11 session. This applies to matching pointer devices, not just one specific mouse. If it causes trouble, remove the file and restart the session again.

First check which session you're using. Run echo $XDG_SESSION_TYPE in a terminal. If it says wayland, the Xorg file won't apply to that session, so you'll need a different method.

If it says x11 but the configuration doesn't help, share your Mint version, desktop environment, mouse model, and any errors. That should make it easier to narrow down what's handling the pointer.