This population-based case-control study in Montréal (2008–2011) evaluated occupational extremely low-frequency magnetic field (ELF-MF) exposure and postmenopausal breast cancer risk using a job-exposure matrix linked to lifetime job histories. Overall, it reports no association between occupational ELF-MF exposure and postmenopausal breast cancer. However, analyses focusing on specific exposure windows (0–10 years before interview or during breast development) reported some positive associations, especially for ER+/PR+ tumours.