Coastal deltas face unprecedented challenges due to accelerating relative sea-level rise, prompting extensive research into mitigating land loss. Traditional models of delta evolution often overlooked the crucial role of mud, focusing primarily on sand deposition while treating mud as washload. Through integrated numerical modeling, physical experiments, and field studies, we investigate the dynamics of mud deposition, particularly in the delta-plain and bottomset regions. Our findings reveal that 1) bottomset aggradation accelerates shoreline progradation by reducing foreset length, thereby influencing delta morphology and evolution, and 2) enhanced mud retention by increasing inter-channel floodplain storages sustains a rapid rate of shoreline progradation compared to previous predictions. This autogenic mechanism, driven by mud capture across the growing delta plain and bottomset, underscores the fundamental role of mud in shaping coastal delta landscapes and sedimentary records. Incorporating these insights into delta evolution models enhances our understanding of coastal dynamics, aiding in interpreting sedimentary records and informing more effective management strategies for vulnerable coastal deltas. Our research highlights the importance of comprehensively considering mud dynamics in coastal delta evolution studies, emphasizing its significance in shaping resilient and sustainable coastal landscapes.