For ten seasons, Lennix co-starred in the role of Harold Cooper, assistant director of the FBI Counterterrorism Division, on the NBC drama The Blacklist (2013-2023).

In film, Lennix has had supporting roles such as The Five Heartbeats (1991), Comfortably Numb (1995), Get on the Bus (1996), Love & Basketball (2000), The Matrix series (1999–2004), Ray (2004), Barbershop 2: Back in Business (2004), Stomp the Yard (2007), and State of Play (2009).

In television, he had a recurring role in Diagnosis: Murder as Agent Ron Wagner as well as a guest voice role in the Legion of Super Heroes episode "Timber Wolf" as the titular character's villainous father Mar Londo, being replaced by Dorian Harewood in the character's later appearance in the episode "Cry Wolf".

He played the parts of Boyd Langton in Joss Whedon's series Dollhouse[6] and U.S. president Barack Obama in the comedy sketch show Little Britain USA.

In July 2014, Lennix formed his own production company Exponent Media Group (EMG) along with long time business partner Steve Harris.

[8] Batman v Superman: Dawn of Justice in 2016 was Lennix' first appearance in a film directed by Zack Snyder, where he played the character Calvin Swanwick.