At the time Fillion appeared on Buffy, he had already portrayed Malcolm Reynolds in Firefly and would go on to reprise the role in the film Serenity. He also starred and sang in Joss Whedon's 2008 Internet musical Dr. Horrible's Sing-Along Blog, along side Felicia Day.
In 2007, Fillion portrayed Dr. Adam Mayfair in Desperate Housewives. From 2009 to 2016, he played the title role on the ABC drama Castle. In 2015 and 2016, Fillion worked with Alan Tudyk on a web series called Con Man, loosely based on their experiences on the convention circuit after Firefly.
In 2017, Fillion was cast in the recurring role of Gary West on the Netflix horror-comedy series Santa Clarita Diet. In the same year, he was cast in the recurring role of Jacques Snicket on the second season of the Netflix comedy drama series A Series of Unfortunate Events.
In 2018, Fillion portrayed John Nolan in ABC's crime-comedy-drama series The Rookie. In 2021, he portrayed the metahuman The Detachable Kid or T.D.K. in The Suicide Squad.
His voice is also featured in animation and video games, such as Hal Jordan/Green Lantern in various DC Comics projects, the Bungie titles Halo 3, Halo 3: ODST, Halo: Reach, Destiny and Destiny 2, along with the 343 Industries video game Halo 5: Guardians.
Fillion special guest starred as the evil priest Caleb in five episodes of Buffy the Vampire Slayer season 7, in one of them also portraying the First Evil in Caleb's form:
- Nathan Fillion on Wikipedia