He is best known for his roles in The CW Arrowverse series Arrow, The Flash, Supergirl, and Batwoman as John Diggle / Spartan, portraying Diggle and Bass Reeves in Legends of Tomorrow, recurring as an alternate universe version of Diggle in Superman & Lois, recurring as Anton Briggs on the Showtime TV series Dexter, and starring in the film Mother and Child (2009) as Joseph.

He has also starred in recurring roles in All of Us, One on One, The West Wing, CSI: Crime Scene Investigation, Ghost Whisperer, Wildfire, and Hollywood Residential.

In 2010, he appeared in an episode of Grey's Anatomy and in the short-lived NBC courtroom drama series Outlaw.

[2] From 2012 to 2020, he appeared in the main cast of CW's superhero show Arrow, as John Diggle, a military veteran who is Oliver's partner, confidant, bodyguard, and later gained an alter ego, Spartan, since the fourth season.

[3] As part of the greater Arrowverse, Ramsey has reprised his role as Diggle on The Flash, Legends of Tomorrow, Supergirl and Batwoman, as well as portraying an alternate version of the character on the non-Arrowverse Superman & Lois.