A first generation prototype, P1, Marx-topology klystron modulator has been developed at the SLAC National Accelerator Laboratory for the International Linear Collider (ILC) project[1]. It is envisioned as a lower cost, smaller footprint, and higher reliability alternative to the present, bouncer-topology, baseline design. The application requires 120 kV ( /-0.5%), 140 A, 1.6 ms pulses at a rate of 5 Hz. The Marx constructs the high voltage pulse by combining, in series, a number of lower voltage cells. The Marx employs solid state elements; IGBTs and diodes, to control the charge, discharge and isolation of the cells. Active compensation of the output is used to achieve the voltage regulation while minimizing the stored energy. The P1-Marx has been integrated into a test stand with a 10 MW L-band klystron, where each is undergoing life testing. A review of the P1-Marx design and its operational history in the L-band test stand are presented. Show Partial Abstract