Since 2007, the global mole fraction of atmospheric methane (CH4) has steadily increased meanwhile the 13C/12C isotopic ratio of CH4 (expressed as δ13C-CH4) has shifted to more negative values. This suggests that CH4 emissions are primarily driven by biogenic sources. However, more in situ isotopic measurements of CH4 are needed at the local scales to identify which biogenic sources dominate CH4 emissions regionally. In California, dairies contribute a substantial amount of CH4 emissions from enteric fermentation and manure management. In this study, we present seasonal atmospheric measurements of δ13C-CH4 from dairy farms in the San Joaquin Valley of California. We used δ13C-CH4 to characterize emissions from enteric fermentation by measuring downwind of cattle housing (e.g., freestall barns, corrals) and from manure management areas (e.g., anaerobic manure lagoons) with a mobile platform equipped with cavity ring-down spectrometers. Across seasons, the δ13C-CH4 from enteric fermentation source areas ranged from -69.7 ± 0.6 per mil (‰) to -51.6 ± 0.1‰ while the δ13C-CH4 from manure lagoons ranged from -49.5 ± 0.1‰ to -40.5 ± 0.2‰. Measurements of δ13C-CH4 of enteric CH4 suggest a greater than 10‰ difference between cattle production groups in accordance with diet. Isotopic signatures of CH4 were used to characterize enteric and manure CH4 from downwind plume sampling of dairies. Our findings show that δ13C-CH4 measurements could improve the attribution of CH4 emissions from dairy sources at scales ranging from individual facilities to regions and help constrain the relative contributions from these different sources of emissions to the CH4 budget.