A system, apparatus, and method are directed to managing automatic sign in to an Instant Messaging (IM) session on mobile device from another computing device, such as a personal computer (PC), over a network. The end-user of the PC may provide an account identifier, password, and the like, and in response, receive a cookie, that may further include the account identifier, encoded string, and the like. The end-user may then request initiation of the IM session using a toolbar selection, a script, applet, form, web page selection, and the like. As part of the request, the cookie may be provided to the server. If the request is authenticated based, in part, on the cookie information, the server uses the account identifier to locate an associated a mobile device identifier. The mobile device identifier is employed to log the end-user automatically into the mobile device.