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Stable isotope ratios of H, C, and O are powerful indicators of a wide variety of planetary geophysical processes, and for Mars they reveal the record of loss of its atmosphere and subsequent interactions with its surface such as carbonate formation. We report in situ measurements of the isotopic ratios of D/H and ¹⁸O/¹⁶O in water and ¹³C/¹²C, ¹⁸O/¹⁶O, ¹⁷O/¹⁶O, and ¹³C¹⁸O/¹²C¹⁶O in carbon dioxide, made in the martian atmosphere at Gale Crater from the Curiosity rover using the Sample Analysis at Mars (SAM)’s tunable laser spectrometer (TLS). Comparison between our measurements in the modern atmosphere and those of martian meteorites such as ALH 84001 implies that the martian reservoirs of CO₂ and H₂O were largely established ~4 billion years ago, but that atmospheric loss or surface interaction may be still ongoing.