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.