Mixpanel's client-side libraries utilize a system we call "super properties" to send user-specific data along with your events. Super properties are no different from normal properties in Mixpanel, except that they are sent with every event for that customer.
Each client-side library maintains a list of super properties that you can write to using a method called register.
Our JS library uses a cookie (created in the domain of the page loading the lib) to store super properties. These are stored as JSON in the cookie. They will persist for the life of that cookie, which by default is 360 days. If you wish to change the life of the cookie, you may do so using set_config to adjust the value for the cookie_expiration(an integer in days).
Android and iOS
Our mobile libraries store your super properties in local storage. They will persist so long as the app is installed (between launches and updates). Uninstalling the app will remove that customers super properties.
Mixpanel's server-side libraries do not automatically append "super properties" to their events. You are more than welcome to roll your own system to append whatever properties you'd like to events for a given user. The most important thing to note when dealing with appending properties server side is that you must include a value for the (traditionally super) property "distinct_id" in order to use the events in most Mixpanel reports. The distinct_id property ties an event to a specific user.