Is there a way to log when and who has logged in ? (Incl userId and timestamp)
Maybe there's something built into Umbraco 7 or above or do I need to make a custom solution for this?
Yes, it's built inside Umbraco. You can found logs in /App_Data/Logs/... directory and there you can find the detailed informations about who and when logged in into the panel.
There are also some packages helping to browse and search inside them directly from backoffice e.g. Diplo Trace Log Viewer (https://our.umbraco.org/projects/developer-tools/diplo-trace-log-viewer/).