The 5G mobile network is embracing new technologies to keep providing network subscribers with a high Quality of Service (QoS). However, this has become increasingly difficult in the urban landscape as more devices are being connected and each device is requesting increasing amounts of data. Network operators rely on the small cell technology to maintain coverage and service for its subscribers, but this technology is incapable of mitigating the increasing workload on the network infrastructure and preventing the associated network delays. The next logical step is to cover the urban landscape with mobile small cells, since these take advantage of the dynamic network topology and optimizes network services in a cost-effective fashion while taking advantage of the high device density. However, the introduction of mobile small cells raises various security challenges. Cryptographic solutions are capable of solving these as long as they are supported by an appropriate key management scheme. In this article, we propose DISTANT: a DIStributed Trusted Authority-based key managemeNT scheme. This key management scheme is specifically designed to provide security in a network which takes advantage of the mobile small cell technology. The scheme relies on threshold secret sharing to decentralize trust and utilizes the self-generated certificates paradigm. Through an extensive security analysis and communication overhead evaluation, we conclude that our design provides an improved level of security and has a low communication overhead compared to previous works.