Mycoplasma bovis causes contagious mastitis and respiratory disease in cattle. Its importance has increased worldwide due to its increasing resistance to antimicrobial agents and the lack of an effective vaccine. Chronically infected silent carriers introduce infection to naive herds, but these carriers can escape detection in current diagnostic assays. The lack of suitable diagnostic tools for extensive screening has hindered appropriate treatment and limited our ability to determine the full impact of this pathogen in cattle industries. We have developed a novel IgG ELISA (MilA ELISA) using a recombinant immunogenic fragment of the MilA protein (encoded by MBOVPG45_0710) and assessed its performance using sera from experimentally infected cattle. We found that the MilA ELISA was able to detect M. bovis-specific antibodies within 3 weeks of infection and had higher diagnostic sensitivity and higher diagnostic specificity than the commercially available ELISAs. We assessed the performance of the MilA ELISA using paired sera from 7448 feedlot cattle and used Bayesian latent class modelling to calculate the globally optimal cut-off for detection of infection with M. bovis in cattle in the field. Using this cut-off, 13.1% of the cattle were seropositive on entry into feedlots and 73.5% were seropositive at follow up approximately six weeks later. We also tested the specificity of the MilA ELISA using serum from calves naturally infected with other bovine mycoplasmas, including M. dispar, M. ovipneumoniae and M. bovirhinis and found the assay to be highly specific for antibodies against M. bovis. Overall these results suggest that this assay is useful as a diagnostic tool for epidemiological investigations to better estimate the impact of M. bovis on animal welfare and productivity. It may also be of use in the development of control measures, which in turn may aid in reducing the development of antimicrobial resistance in this important pathogen.