The Baldwin-Felts Detective Agency played a controversial role in the early years of the coal industry of southern West Virginia, enforcing public law in the coalfields at the direction of the coal operators who hired them. Their often brutal, repressive policies, especially toward union miners, contributed significantly to the violence of the period.

The agency was founded in the early 1890s by William G. Baldwin and Thomas L. Felts in Roanoke, Virginia, initially to contract as a private police force to protect railroads. It soon expanded to the coal industry and opened a second headquarters in Bluefield, West Virginia.

From the beginning, Baldwin-Felts agents were deputized by hard-pressed local sheriffs to maintain law and order, but coal operators increasingly used them to prevent organization of the miners by the United Mine Workers of America. The guards kept union sympathizers from entering the coal camps. They spied on miners, reporting those with union sympathies, their highly organized spy system extending even into the UMWA leadership. In 1902, they helped break a strike in the New River Field. Eventually, their overriding purpose in West Virginia was to prevent unionization of the miners.

More than 150 Baldwin-Felts guards—called "thugs" by the miners—sought to thwart the Paint Creek-Cabin Creek strike of 1912. Union members vigorously resisted. The ensuing violence resulted in the death of several guards, miners, and others.

During a 1920 UMWA organizing campaign along the Tug Fork, the agency again leaped to national attention in the Matewan Massacre of May 19. After the guards had evicted families from their homes, angry miners, led by Police Chief Sid Hatfield, killed seven detectives, including Al and Lee Felts, the founder's brothers. A jury acquitted all of the accused, such was the hatred of the detectives. On August 1, 1921, Baldwin-Felts agents gunned down Hatfield and deputy Ed Chambers at Welch, helping to incite a march on Mingo by more than 10,000 miners in September. The march and ensuing Battle of Blair Mountain were the culminating events of the Mine Wars.

Baldwin-Felts guards also were involved in the 1914 "Ludlow Massacre" in Colorado's coalfields, in which women and children were burned to death in a union tent colony. Three separate congressional investigations of labor violence in West Virginia and Colorado produced much negative publicity about the guard system. Partly because of that, use of Baldwin-Felts guards declined in the 1920s and ended in the 1930s. The agency closed after the deaths of Baldwin in 1936 and Felts a year later.