Bacteriophage DuncansLeg is a siphovirius isolated in 2021 from soil on the Coastal Carolina University campus in Conway, South Carolina, using the host Mycobacterium smegmatis mc2155. DuncansLeg has a 75,593-bp circular genome that contains 126 predicted protein-coding genes and 10 tRNA genes. DuncansLeg is assigned to actinobacteriophage cluster L3.