Disclosed are various embodiments for management of third-party accounts for users in an organization. A request is received from a client corresponding to a user in an organization to access a third-party network site under management by the organization. The third-party network site is operated by a third party that does not correspond to the organization. It is determined whether network traffic between the client and the third-party network site is routed via a proxy server operated by the organization. Access of the client to a managed account with the third-party network site is denied in response to determining that the network traffic between the client and the third-party network site is not routed via the proxy server.