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Abstract To explore both environmental change and the response of non‐fossilizing phytoplankton across the Cretaceous‐Paleogene (K‐Pg) boundary mass extinction event, we determined changes in organic matter (OM) sources using a range of apolar (n‐alkanes, acyclic isoprenoids, steranes, and hopanes) and polar (BIT index) biomarkers. We analyzed two K‐Pg proximal sections, located in the Mississippi Embayment, Gulf Coastal Plain (USA), covering ∼300 kyrs prior to and ∼3 myrs after the K‐Pg event. The OM abundance and composition changed dramatically across the boundary. The post‐impact ejecta layer and burrowed unit are characterized by an increase in the mass accumulation rate (MAR) of plant and soil biomarkers, including high‐molecular‐weightn‐alkanes and C₂₉steranes as well as the BIT index, related to an erosive period which transported terrestrial OM to the ocean in the aftermath of the impact event. At the same time, MARs of putative aquatic biomarkers decrease (low‐molecular‐weightn‐alkanes, C₂₇steranes and pristane and phytane), which suggests a collapse of the marine phytoplankton community. The increase of terrestrial OM to the ocean, during the first 280 kyrs after the Chicxulub impact event, is a combination of reworked kerogen, soil and some plant material. Crucially, within the latter part of this erosion period, only ∼160 kyrs after the K‐Pg do biomarkers return to distributions similar to those in the upper Cretaceous, although not to pre‐impact MARs. Thus, our results suggest a long‐term interval for the full sedimentary and ecological recovery of the non‐fossilizing phytoplankton community after this event.