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We report successful implementation of high-density 3D integration technology in the fabrication of advanced mixed-signal processors for infrared focal plane arrays. Separate analog and digital integrated circuits (ICs) were custom designed and fabricated in standard bulk CMOS technology in two different commercial foundries. The 3D interconnects were arrayed in a 256×256 format with a 30 micron pitch and had the form of through-silicon vias 4 micron in diameter and 30 micron deep. Completed 3D readout IC stacks were tested pixel by pixel, demonstrating operability of 99.9%. The 3D IC stacks were hybridized with infrared photodiode arrays and produced functional FPA imagers with 99.9% array operability and unprecedented performance. Such imaging arrays and other smart sensors enabled by 3D integration of sensing elements with sophisticated signal processing are of increasing interest for Internet-of-Things applications, from net-enabled surveillance to mobile health.