This paper presents the electrical and mechanical design considerations of a compact, 160 J modular pulse forming network (PFN) based Marx generator used to drive a high-power microwave (HPM) source with a time variant load at a PRF of 100 Hz. The modular Marx generator is designed to produce an open circuit output voltage of 600 kV from a 50 kV source using twelve stages. Each stage of the Marx was constructed from a PFN created with five, 2.1 nF, high voltage capacitors in parallel. Each Marx module was machined out of acetyl copolymer or Delrin to provide optimal strength, rigidity, and a dielectric constant that closely matches transformer oil. These Marx modules include air supply lines that are machined directly into each block of Delrin allowing airlines to connect to each module chamber rather than every spark gap. The spark gaps are comprised of two electrode inserts placed into the sealed pressure vessel contained within the Marx modules. The electrode inserts are of a sleeve-electrode design, which allows the user to insert the electrode into the sleeve, then the sleeve into the Marx module. The benefit of this design is the ability to adjust the electrode gap spacing without compromising the high pressure seal. Two continuous charging inductors run between each PFN and underneath the Marx modules. Due to high voltages generated by the Marx, outer field shaping rings are used to reduce the field stress across the induction coils, resulting in longer lifetime. The inductors are also of modular design allowing for individual coils to be replaced in the event of failure. Output voltage and current waveforms from a 60 Ohm water load are presented.