Buffy the Vampire SlayerEdit
DeKnight's first collaboration with Joss Whedon was on Buffy the Vampire Slayer where he wrote 5 episodes during the fifth and sixth season. He also served as a story editor during the show's sixth season, and wrote issue 22, titled "Swell", of the currently ongoing Season Eight.
After Buffy's sixth season DeKnight joined Angel's fourth season as a writer and co-producer, becoming a producer in season five, and eventually ending up as supervising producer for the last nine episodes. This move was motivated by Tim Minear's departure for Firefly. DeKnight wrote or co-wrote 13 Angel episodes during those last two years, directing three of them, and becoming one of the most prolific Angel writers.
After Angel's cancellation DeKnight joined the staff of Smallville as a writer, director and supervising producer in its fourth season. He wrote fifteen episodes in seasons 4 through 6, directing two of them, and being an co-executive producer for most of this time. And extensive two-part interview about his tenure on Smallville can be found here: Part 1, Part 2.
DeKnight was announced alongside Tim Minear as a consulting producer of Dollhouse at the Fox Upfronts in May 2008. He has written and directed the second episode "The Target", departing from Dollhouse soon after finishing its production. On October 26th, 2008 Joss Whedon announced that DeKnight has ended his consulting duties: "And finally, young Steve DeKnight, after writing and shooting an ep so cool it helped not only define the show but save its ass, is ending his consulting duties, the f#%&er." 
Shortly after his departure from the Dollhouse staff, DeKnight joined the production of Spartacus as head writer and show-runner.