Echo Kellum has been cast as Mr. Terrific. To quote Entertainment Weekly,
"The Ben and Kate alum will play Curtis Holt, aka Mr. Terrific. Holt is a technological savant and inventor who works at Palmer Technologies under the supervision of Felicity Smoak (Emily Bett Rickards)."
It was also confirmed at Comic-Con that this version of Mr. Terrific will be gay. It is still unknown if Kellum will be a series regular or a recurring character. Neal McDonough is also joining the cast as Damien Darhk this season, as well as Anarky, who has not been cast yet.
In the comics, Mr. Terrific is the third smartest human on earth, (just behind Ray Palmer) and a long-time member of the Justice Society of America.