This paper reports the development of a compact and highly programmable 28-GHz phased array subsystem that is currently being integrated in a city-scale testbed for advanced wireless research and experimentation. The subsystem hardware consists of a 28-GHz 64-element state-of-the-art dual-polarized phased array antenna module (PAAM), low-noise DC-DC power regulators with a single 12-V input, current sensors and an ADC for real-time supply monitoring, a PLL for LO (5 GHz) generation, IF (3 GHz)/LO splitters, IF/LO baluns, and programmable switches. All of these components are integrated in a compact (10' x5.75') PCB which also supports the direct plug-in of a commercial-off-the-shelf FPGA system-on-module (SoM). The IF switches and splitters couple the PAAM to 20 IF ports that enable a rich set of MIMO configurations, including up to 8 simultaneous 16-element independent beams in each TX/RX mode. The FPGA and associated software enable the configuration of all subsystem features from a high-level application program interface (API), facilitating MIMO millimeter-wave wireless experiments. 28-GHz channel sounding measurements using the phased array subsystem coupled with software-defined radios is presented as a testbed-level experiment example.