Several stalagmite records have yielded important but discontinuous insights into northern Iberian climate since the Last Glacial. Here we present the first continuous Iberian stalagmite-based reconstruction of climate since the Bølling-Allerød interstadial, from a single stalagmite sample (GAR-01 from La Garma Cave, Cantabria). The ∼13.5 ka GAR-01 record provides the opportunity for replication, continuation, and aggregation of previously published records from northern Spain. The GAR-01 record reveals shifts in oxygen isotope ratios that are inexplicable by appealing to a single control (i.e., exclusively temperature, rainfall amount, etc.). Herein we explore the potential role of rainfall and temperature seasonality shifts on the new δ18O record using a simple Monte Carlo approach to estimate the seasonal distribution of rainfall and the annual temperature range at 100-year timeslices across the record. This model is corroborated by intervals of monthly-resolved laser ablation trace element data, providing glimpses into past Iberian seasonality shifts. The most salient features of the modelled results include extremely dry Younger Dryas winters (∼12.9–11.6 ka BP) and several intervals during the mid-Holocene with almost no summer rainfall (e.g., at 4.2 and 9.0 ka BP). By 1.6 ka BP, a near-modern rainfall seasonality was established. According to the modelling results, seasonal rainfall and temperature distribution variability can account for 95% of the record. The model presented here provides a new tool for extracting critical missing seasonality information from stalagmite δ18O records. Intervals where the model does not converge may represent transient climate anomalies with unusual origins that warrant further investigation.