The recently confirmed Hapcheon impact crater in Korea is a complex impact structure 7 km in diameter, with clear rims and impact-driven underground lacustrine sedimentary features. We investigated the lithological features of deposits within the impact crater using drilled sedimentary cores (23HIC01, 20CR05, 20CR09, and 20CR10), which consisted of an ascending order of impact breccias, lake sediments, and subaerial (e.g., wetland) sediments. The impact breccia deposits in the 20CR05 and 23HIC01 cores contain shatter cones, which are a macroscopic indication of a meteorite impact. The overlying lake sediments were divided into three stages. The early stage of the post-impact lake environment corresponded to the lowermost lake sediments with frequent microfaults and slump-turbidite events. This stage is characterized by high calcite content of up to 13%. The middle stage showed a stable depositional environment, with silty to sandy lamination and bedding, and fewer microfaults. The final stage of the post-impact lake environment appears to have been very short and dramatic. This ended with the final slumping event, which appears to have been triggered by an abrupt outburst of lake water. This study demonstrates early post-impact lake sedimentation processes and crater instability in terms of soft-sediment deformation structures (e.g., microfaults and slumps).