Magnesium oxychloride cement (MOC) boards have the potential to offset carbon emissions through mineral carbonation, a process whereby carbon dioxide (CO2) is converted to carbonate minerals. Boards (0-15 years old) contained MOC phase 5 (21-50 wt.%), brucite, primary (e.g., magnesite) and secondary (hydromagnesite and chlorartinite) carbonate minerals. Quantitative mineralogy, electron microscopy and carbon abundance data demonstrate that secondary carbonates form through the reactions of MOC and brucite with CO2 within interfacial water layers after board manufacturing. Stable carbon isotopic data confirmed the source of sequestered CO2 as being the atmosphere. Carbonation rates were approximately 0.07 kg CO2/m2 board/year or 9 kg CO2/t board/year over 15 years, offsetting ~20-40% of estimated carbon emissions. In experiments using 10% and 100% CO2 gas, carbonation was accelerated by approximately 400 and 1600 times in comparison to the passive rate. Integration of carbonation reactions into MOC board production could provide significant carbon offsets.