This paper presents a robust control approach for non-isolated Modular Multiport Converters (MMPC) capable of integrating multiple energy sources and loads. The objective of this robust control approach is to mitigate cross-coupling challenges inherent in MIMO systems and effectively manage the parametric uncertainties associated with the converter as well as input and output disturbances. To achieve this objective, the paper begins with deriving the general nonlinear dynamic equations of an n-level step-up multi-port DC/DC converter (MPDCnLSU). Subsequently, for a case study involving a 3-level step-up multi-port DC/DC converter (SMPDC3LSU) the equations are linearized to obtain the state-space model. Following the derivation of the converter model, a controller comprising two control loops is designed. The outer loop, responsible for regulating the voltage of output ports, is synthesized through a robust μ-optimal method using the D-G-K iterative procedure, while the inner loop, responsible for regulating the current sharing among the parallel modules and generating PWM signals, is stabilized via multiple PI controllers. Finally, hardware-in-the-loop (HIL) test results derived from OPAL-RT 4610, and experimental results from a prototype are used to validate this control approach. The proposed decoupled mixed μ synthesis method ensures robust performance and stability and results in a less conservative controller design for the MPDC3LSU.@en