This paper presents a millimeter-wave (mmwave) multiple-input multiple-output (MIMO) antenna for the fifth-generation (5G) networks and beyond. The proposed antenna frontend aims to deliver a high gain and bandwidth, planar configuration for a compact assembly, and high isolation for the MIMO operation. The top layer is comprised of a modified T-shaped radiating patch integrated with a coplanar waveguide (CPW) feed on top of a substrate Rogers RT Duroid 5880. The bottom partial ground layer is comprised of a series of six ring-slots as the defective ground structures (DGS) in a symmetrical triangular formation at optimized positions. The results show an impedance bandwidth in the range of 24.5-38.4 GHz with a maximum gain of 6.19 dBi at 36.75 GHz. The 2×2 MIMO assembly is presented with a lower mutual coupling and transmission coefficient below -20 dB. The performance of the proposed MIMO antenna is on merit to support various mm-wave wireless applications such as cellular phones and indoor base stations for 5G and beyond.