AZ91/B4C metal matrix composites (MMCs) comprising three different amounts of B4C particulates, 12, 18, and 24 wt% were fabricated using a melt stirring-squeeze casting technique. Machinability studies were conducted on the cast composites using uncoated and CVD diamond coated WC-Co inserts. The tool life of the uncoated WC-Co and CVD diamond coated inserts was calculated at different cutting speeds, 30, 60, and 90 m/min. Results revealed that with the increasing cutting speeds, the surface roughness of the MMCs decreased and the magnitude of the main cutting forces increased. The increased magnitude in the cutting speed and particle proportion significantly reduced the tool life. The abrasive action of the hard B4C particles caused severe tool wear during the machining of AZ91–12%B4C, AZ91–18%B4C, and AZ91–24%B4C composite materials. From the tool flank wear results, it was evident that diamond coated inserts outperformed uncoated WC-Co inserts at all machining conditions. The tool life of the CVD diamond coated insert at the cutting speed of 30 m/min was found to be the highest.