The main drivers contributing to the continued growth of network traffic include video, mobile broadband and machine-to-machine communication (Internet of Things, cloud computing, etc.). Two primary technologies that next-generation (5G) networks are using to increase capacity to meet these future demands are massive MIMO (Multi-Input Multi-Output) antenna arrays and new frequency spectrum. The massive MIMO antenna arrays have significant thermal challenges due to the presence of large arrays of active antenna elements coupled with a reliance on natural convection cooling using vertical plate-finned heat sinks. The geometry of vertical plate-finned heat sinks can be optimized (for example, by choosing the fin pitch and thickness that minimize the thermal resistance of the heat sink to ambient air) and enhanced (for example, by embedding heat pipes within the base to improve heat spreading) to improve convective heat transfer. However, heat transfer performance often suffers as the sensible heat rise of the air flowing through the heat sink can be significant, particularly near the top of the heat sink; this issue can be especially problematic for the relatively large or high-aspect-ratio heat sinks associated with massive MIMO arrays. In this study a vertical plate-finned natural convection heat sink was modified by partitioning the heat sink along its length into distinct sections, where each partitioned section ejects heated air and entrains cooler air. This approach increases overall heat sink effectiveness as the net sensible heat rise of the air in any partitioned section is less than that observed in the unpartitioned heat sink. Experiments were performed using a standard heat sink and equivalent heat sinks partitioned into two and three sections for the cases of ducted and un-ducted natural convection with a uniform heat load applied to the rear of the heat sink. Numerical models were developed which compare well to the experimental results and observed trends. The numerical models also provide additional insight regarding the airflow and thermal performance of the partitioned heat sinks. The combined experimental and numerical results show that for relatively tall natural convection cooled heat sinks, the partitioning approach significantly improves convective heat transfer and overall heat sink effectiveness.