The Mackenzie River Basin (MRB) delivers large quantities of organic carbon (OC) into the Arctic Ocean, with significant implications for the global C budgets and ocean biogeochemistry. The amount and properties of OC in the Mackenzie River's delta have been well monitored in the last decade, but the spatial variability in OC sources transported by its different tributaries is still unclear. Here we present new data on the radiocarbon (C-14) content of dissolved and particulate OC (Delta C-14-DOC and Delta C-14-POC) across the mainstem and major tributaries of the MRB, comprising 19 different locations, to identify factors controlling spatial patterns in riverine OC sources. The Delta C-14-DOC and Delta C-14-POC varied across a large range, from -179.9 parts per thousand to 62.9 parts per thousand, and -728.8 parts per thousand to -9.0 parts per thousand, respectively. Our data reveal a positive spatial coupling between the Delta C-14 of DOC and POC across the MRB, whereby the most C-14-depleted waters were issued from the mountainous west bank of the MRB. This C-14-depleted DOC and POC likely originates from a combination of petrogenic sources, connected with the presence of kerogens in the bedrock, and biogenic sources, mobilized by thawing permafrost. Our analysis also reveals intriguing relationships between Delta C-14 of DOC and POC with turbidity, water stable isotope ratio and catchment elevation, indicating that hydrology and geomorphology are key to understanding riverine OC sources in this landscape. A closer examination of the specific mechanisms giving rise to these relationships is recommended. For now, this study provides a road map of the key OC sources in this rapidly changing river basin.