To better understand the clinical pathology, diseases, and causes of mortality of reintroduced American martens (Martes americana) in Michigan, a study was conducted from 2011 to 2015 in the Upper and Lower Peninsulas of Michigan. Samples obtained from live trapping (n = 58) or harvested carcasses (n = 34) were serologically tested for select pathogens. Antibodies against Toxoplasma gondii and canine distemper virus were detected in 58 and 3.4% of samples, respectively. All samples were seronegative for Leptospira spp. and negative for Dirofilaria immitis antigen. Urine samples tested for Leptospira spp. via immunofluorescent antibody assay (n = 7), polymerase chain reaction (n = 6), or both (n = 3) were all negative. Parvovirus DNA was detected in 9.1% of small intestine samples (n = 22) collected from carcasses and in 3.7% of fecal samples (n = 27) collected during live trapping. Complete blood counts (n = 64) and serum biochemistries (n = 63) were obtained from 49 live-trapped martens. Biochemical parameters found to be significantly different (P < 0.05) between genders were calcium, creatinine, glucose, and phosphorus. There was no significant difference between genders for any hematologic parameter. Significant differences (P < 0.05) between summer and winter seasons were found in total estimated white blood cell count, neutrophils, lymphocytes, monocytes, alkaline phosphatase, bicarbonate, calcium, creatinine, globulin, glucose, phosphorus, potassium, sodium, and total protein. There was no significant difference in blood cell count or serum biochemistry values between radio-collared (n = 17) and noncollared (n = 47) martens. Animals seropositive for T. gondii were found to have significantly higher (P < 0.05) eosinophil and globulin levels than seronegative animals. The primary natural cause for mortality of radio-collared American martens was predation. Histologic examinations revealed a high percentage (60%) of martens with verminous or granulomatous pneumonia.