Prosthetic joint infections (PJI) complicate up to 2% of arthroplasties and are usually caused by typical bacterial agents (e.g., staphylococci and streptococci). Although an infrequent cause of PJI, mycobacterial species are difficult to eradicate, as they adhere to hardware, form biofilms, and have high rates of antimicrobial resistance. Mycobacterium mageritense is a rapidly growing Mycobacterium that has been infrequently described as a cause of surgical and device-related infections. We herein described a case of prosthetic knee infection due to M. mageritense. The patient was treated with removal of hardware, antimicrobials, and prosthetic knee reimplantation with a good outcome. To our knowledge, M. mageritense has not been previously described as a cause of PJI in the medical literature.