Phosphorus (P) supply is pivotal to global ecological functioning and carbon cycling. Ombrotrophic peatlands are waterlogged, nutrient-limited ecosystems where plant litter production exceeds decomposition rates, creating an important soil carbon (C) store (~500 Pg). Published experimental evidence demonstrates that the tight stoichiometric balance between P, nitrogen (N) and C regulates present-day peatland behaviour and carbon sequestration. We know little about the dynamics and importance of P in long-term bog development, limiting understanding of how the global peatland carbon sink may respond to prolonged human-perturbed and elevated P deposition. Here we show that P supply imposes a strong and overlooked control on long-term biogeochemical cycling and carbon storage across a range of individual bogs. Holocene stoichometric (P, N and C) profiles from 23 ombrotrophic, Sphagnum peatlands across Europe, North and South America confirm the importance of P recycling and retention by living vegetation in the acrotelm. Phosphorus accumulation in the catotelm shows a strong, positive correlation with site-specific proxies for decomposition (r2 = 0.91) and the P:N balance appears linked to long-term C burial (r2 = 0.63). More significantly, we find that modern measured P deposition in each study region is associated with greater decomposition and lower carbon accumulation in the catotelm. As ombrotrophic bogs source P solely from the atmosphere, the P supply to a particular bog appears to drive its biogeochemical cascade from surface vegetation to catotelm storage. We propose, supported by published experimental evidence, that P constrains productivity and microbial nutrient turnover over Holocene timescales. In turn, this dictates the intensity of organic matter decomposition, the amount of C export prior to permanent burial and, ultimately, the size of globally significant carbon and nutrient stores. We also calculate the P pool in mid-latitude ombrotrophic peatlands to be 0.23 Gt, representing 1.7% of global soil P. Our findings identify P supply as an overlooked driver of long-term peatland development and raises the prospect that increased post-industrial P deposition may degrade the global peatland carbon sink.