He is the founder of Mutant Enemy Productions, co-founder of Bellwether Pictures, and is best known as the creator of several television series: the supernatural drama Buffy the Vampire Slayer (1997–2003) and its spinoff Angel (1999–2004), the short-lived space Western Firefly (2002), the Internet musical miniseries Dr. Horrible's Sing-Along Blog (2008), the science fiction drama Dollhouse (2009–2010), the Marvel Cinematic Universe series Agents of S.H.I.E.L.D. (2013–2020), and the science fiction drama The Nevers (2021).