His first script contribution was co-writing, with co-producer Ben Sokolowski, the third-season episode "The Secret Origin of Felicity Smoak".
Sullivan and Sokolowski co-wrote a teleplay, based on a story by co-executive producer Jake Coburn for the nineteenth installment, "Broken Arrow"; which saw the departure of series regular Colton Haynes (Roy Harper/Arsenal).
During the 2015 Winter Television Critics Association tour, The CW announced a new 6-part animated series, Vixen, centered on the DC heroine, that premiered in August of the year.
The series is written by Keto Shimizu, Sullivan, Arrow showrunner Wendy Mericle, and comic book writer Lauren Certo.
It featured an anticipated guest appearance by John Constantine (Matt Ryan), from the canceled NBC series.