The launch of NuSTAR and the increasing number of binary black hole (BBH) mergers detected through gravitational wave (GW) observations have exponentially advanced our understanding of black holes. Despite the simplicity owed to being fully described by their mass and angular momentum, black holes have remained mysterious laboratories that probe the most extreme environments in the Universe. While significant progress has been made in the recent decade, the distribution of spin in black holes has not yet been understood. Exploring the spin distribution across stellar-mass black holes provides an insight into the formation of black holes, supernova events, collapsar models, gamma-ray bursts, the formation and evolution of X-ray binary systems and binary black hole systems, and the physics of accretion. The preferred spin measurement techniques are “continuum fitting” (see e.g., Gou et al. 2009) and “relativistic reflection” (see e.g., Miller 2007). Relativistic reflection is independent of black hole mass, accretion rate, and distance to the system, making it a more versatile technique. Up until now, spin measurements for the same black hole using the two methods did not always agree, and even measurements using the same method did not always adopt the same sets of assumptions and theoretical prescriptions. My work provides a pipeline that uses state of the art relativistic reflection models to fully explore the entire physical parameter space and to provide a uniform treatment of a large sample of black holes. Using this pipeline on NuSTAR data, I measured more than a dozen new black hole spins in X-ray binary systems, significantly expanding the measured sample size, and remeasured existing black hole spins in order to compile a distribution of measurements made using entirely consistent methods and systematic uncertainties. Additionally, I compared the distribution to that of spins measured in mergers of binary black hole systems observed through gravitational waves, in order to offer a unified view of black hole spin evolution. The results suggest that the observed spin distribution of black holes in X-ray binaries is sharply peaked at high values, incompatible with the spin distribution inferred based on GW signals from merging BBH, suggesting that the observed black hole distributions are inherently different. Understanding the distribution of black hole spins in X-ray binaries paves the way for future Gravitational Wave efforts and for future X-ray missions such as XRISM, HEX-P, AXIS, STROBE-X, or ATHENA.