With Greg Berlanti, Guggenheim is the co-creator of the ABC show "Eli Stone". He later became executive producer of ABC's "No Ordinary Family". Guggenheim, together with Berlanti and Andrew Kreisberg, adapted the Green Arrow comics into the television series "Arrow". The three, alongside Phil Klemmer, went on to develop the spin-off series "Legends of Tomorrow". Guggenheim served as co-showrunner on "Arrow" for seasons 1-6, and on "Legends of Tomorrow" for seasons 1-4. From the fall of 2018, he stepped down as showrunner and serve as an executive consultant to both shows.