Processors are an essential component in most satellite payload electronics and handle a variety of functions including command handling and data processing. There is growing interest in implementing soft processors on commercial FPGAs within satellites. Commercial FPGAs offer reconfigurability, large logic density, and I/O bandwidth; however, they are sensitive to ionizing radiation and systems developed for space must implement single-event upset mitigation to operate reliably. This paper investigates the improvements in reliability of a LEON3 soft processor operating on a SRAM-based FPGA when using triple-modular redundancy and other processor-specific mitigation techniques. The improvements in reliability provided by these techniques are validated with both fault injection and heavy ion radiation tests. The fault injection experiments indicate an improvement of 51× and the radiation testing results demonstrate an average improvement of 10×. Orbit failure rate estimations were computed and suggest that the TMR LEON3 processor has a mean-time to failure of over 76 years in a geosynchronous orbit.