High-entropy alloys (HEAs) are a new class of alloys having equiatomic ratios of alloying elements in their composition. Boron is an important element that can increase the strength of steels and wear resistance of hard facing coatings with its high hardness compounds. The effect of boron (B) in FeNiCoCrCu-based HEAs has not been studied in detail to date. In the current study, the microstructural and mechanical properties of FeNiCoCrCuBx HEAs with varying boron contents (x = 1, 2, 3, 4, 5 at. pct) were investigated using specimens prepared by two-stage processing (sintering and vacuum arc melting). Microstructural and structural studies were carried out using optical microscopy, scanning electron microscopy, electron probe microanalysis, X-ray diffraction, and differential scanning calorimetry. Vickers microhardness and three-point bending tests were also performed to observe the variations in mechanical properties. Results showed that the microstructures of HEAs are generally dendritic and contain two different FCC phases, i.e., Fe-Ni-Co-Cr-rich dendritic and Cu-rich interdendritic phases. The hardness values increased with increasing B content with a maximum hardness of 337 HV. According to three-point bending test results, the highest strength of about 1900 MPa and good ductility were obtained with HEA-3 (3 at. pct B).