Examples pertaining to user equipment (UE) key derivation at mobility update in mobile communications are described. An apparatus implemented in a UE conducts a registration with a first Access and Mobility Management Function (AMF) of a network. The apparatus also conducts one other registration with a second AMF of the network over a 3rd Generation Partnership Project (3GPP) access. In response, the apparatus receives a request from the second AMF over the 3GPP access. The apparatus then performs a key derivation to generate a derived key using a keying material from the initial registration with the first AMF to result in the UE and the second AMF sharing a common security context for the 3GPP access and a non-3GPP access.