The MUltiplexed SQUID/TES Array at Ninety GHz (MUSTANG) is a 64-pixel array of transition-edge sensor (TES) bolometers built at the University of Pennsylvania (UPenn) for the Green Bank Telescope (GBT) in collaboration with a number of universities and government agencies such as NASA-GSFC, NRAO, and NIST. MUSTANG carried out observations between 2008 and 2013 and will soon be replaced by a new receiver (MUSTANG-1.5). MUSTANG-1.5 is a 223-pixel array of feedhorn-coupled polarimeters, which are read out with a novel microwave SQUID multiplexer. MUSTANG-1.5 offers many advantages over MUSTANG including more stable cryogenics, a larger field of view (5.5' compared to 42" for MUSTANG), and a significant improvement in sensitivity. These capabilities enable a far more comprehensive observing program. MUSTANG is aimed at measuring the distortion in the Cosmic Microwave Background (CMB) spectrum that arises due to the Sunyaev-Zel'dovich Effect (SZE). The SZE is the inverse Compton-scattering of CMB photons as they pass through the dense plasma in clusters of galaxies. The SZE is a nearly redshift-independent, complementary probe of the ICM to X-ray emission and combined analyses of both data sets provide a better understanding of astrophysical phenomena such as shocks, cold fronts, and Active Galactic Nucleus (AGN) outbursts. Understanding how substructure, especially in merging clusters, affects the scaling between SZE flux and total cluster mass is essential to placing tight constraints on cosmological parameters with SZE surveys. In this thesis, I present some of the last ever observations carried out by MUSTANG, which are centered on two massive galaxy clusters, MACS J0647.7+7015 and MACS J1206.2-0847. I discuss a model-fitting technique that has been used to quantitatively compare MUSTANG and lower resolution SZE data from Bolocam to study ICM pressure profiles. I report on the design, commissioning, and current status of MUSTANG-1.5 including the detectors, cryogenics, optics, and the microwave multiplexing readout electronics. Finally, I present early characterization of the cryogenics and readout electronics as the instrument nears deployment-readiness and discuss plans for the early science program.