Extrachromosomal, circular DNA (ecDNA) is a prevalent oncogenic alteration in cancer genomes, often associated with aggressive tumor behavior and poor patient outcome. While previous studies proposed a chromatin-based mobile enhancer model for ecDNA-driven oncogenesis, its precise mechanism and impact remains unclear across diverse cancer types. Our study, utilizing advanced multi-omics profiling, epigenetic editing, and imaging approaches in three cancer models, reveals that ecDNA hubs are an integrated part of nuclear condensates and exhibit cancer-type specific chromatin connectivity. Epigenetic silencing of the ecDNA-specific regulatory modules or chemically disrupting liquid-liquid phase separation breaks down ecDNA hubs, displaces MED1 co-activator binding, inhibits oncogenic transcription, and promotes cell death. These findings substantiate the trans -activator function of ecDNA and underscore a structural mechanism driving oncogenesis. This refined understanding expands our views of oncogene regulation and opens potential avenues for novel therapeutic strategies in cancer treatment.