Alterations and unpredictability of the network topology in mobile ad hoc networks (MANETs) are less capable of ensuring the security of multicast data transmissions than in conventional networks. Despite the recent development of many key agreement protocols for MANETs, to our knowledge, only a few secure multicast data transmissions have been integrated into the key agreement. This study proposes a dynamic multicast height balanced group key agreement (DMHBGKA) that allows a user in a multicast group to efficiently and dynamically compose the group key and securely deliver multicast data from a multicast source to the other multicast group users in wireless ad hoc networks. The hierarchical structure of the proposed key agreement partitions the group members into location-based clusters capable of reducing the cost of communication and key management when member joins or leave networks. Moreover, based on elliptic curve Diffie-Hellman (ECDH) cryptography key management, the proposed scheme not only provides effective and efficient dynamic group key reconstructions and secures multicast data transmissions but also fits the robustness of the wireless networks and lowers overhead costs of security management.