The last deglaciation was characterized by a series of millennial‐scale climate events that have been linked to deep ocean variability. While often implied in interpretations, few direct constraints exist on circulation changes at mid‐depths. Here we provide new constraints on the variability of deglacial mid‐depth circulation using combined radiocarbon and neodymium isotopes in 24 North Atlantic deep‐sea corals. Their aragonite skeletons have been dated by uranium‐series, providing absolute ages and the resolution to record centennial‐scale changes, while transects spanning the lifetime of a single coral allow subcentennial tracer reconstruction. Our results reveal that rapid fluctuations of water mass sourcing and radiocarbon affected the mid‐depth water column (1.7–2.5 km) on timescales of less than 100 years during the latter half of Heinrich Stadial 1. The neodymium isotopic variability (−14.5 to −11.0) ranges from the composition of the modern northern‐sourced waters towards more radiogenic compositions, suggesting the presence of a greater southern‐sourced component at some times. However, in detail, simple two‐component mixing between well‐ventilated northern‐sourced and radiocarbon‐depleted southern‐sourced water masses cannot explain all our data. Instead, corals from ~15.0 ka and ~15.8 ka may record variability between southern‐sourced intermediate waters and radiocarbon‐depleted northern‐sourced waters, unless there was a major shift in the neodymium isotopic composition of the northern end‐member. In order to explain the rapid shift towards the most depleted radiocarbon values at ~15.4 ka, we suggest a different mixing scenario involving either radiocarbon‐depleted deep water from the Greenland‐Iceland‐Norwegian Seas or a southern‐sourced deep water mass. Since these mid‐depth changes preceded the Bolling‐Allerod warming and were apparently unaccompanied by changes in the deep Atlantic, they may indicate an important role for the intermediate ocean in the early deglacial climate evolution. Key Points Deep‐sea corals record rapid variability in the mid‐depth NW Atlantic OceanCentennial fluctuations of sourcing and radiocarbon during Heinrich Stadial 1Potential role for the intermediate ocean in early deglacial climate evolution