Radiocarbon (¹⁴C) is a powerful tracer of fossil emissions because fossil fuels are entirely depleted in ¹⁴C, but observations of ¹⁴CO₂ and especially ¹⁴CH₄ in urban regions are sparse. We present the first observations of ¹⁴C in both methane (CH₄) and carbon dioxide (CO₂) in an urban area (London) using a recently developed sampling system. We find that the fossil fraction of CH₄ and the atmospheric concentration of fossil CO₂ are consistently higher than simulated values using the atmospheric dispersion model NAME coupled with emission inventories. Observed net biospheric uptake in June–July is not well correlated with simulations using the SMURF model with NAME. The results show the partitioning of fossil and biospheric CO₂ and CH₄ in cities can be evaluated and improved with ¹⁴C observations when the nuclear power plants influence is negligible.