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We report the fabrication and experimental characterization of a carbon nanotube (CNT)-based MEMS/NEMS electron impact gas ionizer with an integrated extractor gate for portable mass spectrometry. The ionizer achieves low-voltage ionization using sparse forests of plasma-enhanced chemical-vapor-deposited CNTs as field emitters and a proximal extractor grid with apertures aligned to the CNT forests to facilitate electron transmission. The extractor gate is integrated to the ionizer using a high-voltage MEMS packaging technology based on Si springs defined by deep reactive ion etching. The ionizer also includes a high-aspect-ratio silicon structure (??foam) that facilitates sparse CNT growth and also enables uniform current emission. The devices were tested as field emitters in high vacuum (10[superscript -8] torr) and as electron impact ionizers using argon at pressures of up to 21 mtorr. The experimental data show that the MEMS extractor gate transmits up to 66% of the emitted current and that the ionizers are able to produce up to 0.139 mA of ion current with up to 19% ionization efficiency while consuming 0.39 W.