The TITAN facility at TRIUMF is a series of ion traps designed for precision mass spectrometry on rare isotopes. The combination of an on-line electron beam ion trap charge breeder with a Penning trap enables measurements with radioactive ions in high charge states. The use of highly charged ions (HCI) can yield a significant gain in mass precision and mass resolving power. However, the benefits of high charge states are mitigated since the charge breeding deteriorates the beam quality. To achieve suitable beam properties and access the full potential of Penning trap mass spectrometry with HCI a cooler Penning trap (CPET) for electron cooling of highly charged radioisotopes is being developed. In this device short-lived HCI will be sympathetically cooled by a co-trapped electron plasma prior to mass measurement. For electron plasma generation electrons are injected from an off-axis electron gun placed in the fringe field of CPET’s solenoid magnet. We report on the development of an electron gun design that is adapted to the operation in lateral magnetic fields and has enabled efficient and robust electron plasma formation in CPET.