Methods and systems are directed toward enabling a user of a target device, such as a mobile device, to register for automatic initiation of a messaging service on the target device. A target device user interface, such as a login user interface, provides a selectable activation element for the user to indicate that future messaging sessions with the target device should be established automatically. An activation indicator and target identifier are associated with a user identifier and stored in a messaging system data store. When the user ceases use of a messaging session on another device, such as a general purpose computing device, the messaging system uses the user identifier and target identifier to automatically establish a corresponding messaging session on the target device. The user of the target device may then hold a messaging session with an existing third party device or with a different third party device.