Significance Adult hematopoietic stem and progenitor cells (HSPCs) develop from a small number of specialized endothelial cells in the embryo. Very little is known about how this process, known as the endothelial-to-hematopoietic transition, is regulated. In this paper, we used mouse genetic knockout models to establish Chd1 as the first chromatin remodeler, to our knowledge, shown to regulate this transition. Chd1 is not required in the endothelium prior to the transition, nor in the blood system after the transition. We found that the emergence of HSPCs involves an increase in total nascent transcription that is dependent on Chd1. These results reveal a new paradigm of regulation of a developmental transition by modulation of transcriptional output that may be relevant in other stem/progenitor cell contexts.