The electric power sector is undergoing a fundamental transformation driven by the rapid adoption of cost-effective renewable energy generation and the emergence of high-demand applications such as EV fast charging, AI data centers, and manufacturing onshoring. These drivers have reversed a decades-long trend of declining growth in peak demand and disrupted traditional load profiles, placing unprecedented stress on the aging power infrastructure. The resulting grid instability concerns and prolonged connection delays are accelerating a shift toward distributed, flexible, and resilient microgrid deployment models where generation and consumption are co-located. However, traditional microgrids must contend with existing power electronics and use separate single-function converters for each connection point, leading to a complex, costly, and inefficient power conversion architecture. To address these limitations, this dissertation proposes two novel families of flexible multiport power converters (MPCs) capable of interfacing and coordinating multiple sources and loads within a single-stage conversion structure. The Multiport Soft-Switching Solid-State Transformer (MS4T) realizes partially-isolated MPCs, while the Multiport Soft-Switching Current Source Inverter (MSSCSI) achieves non-isolated MPCs with increased power density. Both families support an arbitrary number of AC or DC ports with bidirectional power flow and offer high and symmetrical conversion efficiency enabled by four-quadrant Zero-Voltage Switching (ZVS). New control techniques and modulation strategies are developed to ensure robust soft-switching operation under multiport, four-quadrant conditions. Experimental validations are provided, including demonstrations in DC-DC-DC and DC-DC-3ϕAC configurations. A comprehensive optimization framework is presented to select the Pareto-optimal resonant elements across the multiport application space. Finally, a scalable system architecture based on MS4T and MSSCSI building blocks is proposed and demonstrated through simulation of a representative 500kW multiport system, to support the next generation of flexible, high-performance multiport power electronics for modern microgrids.