He replaces managing partner Glenn Whitney who will be focusing on a European remit incorporating Frankfurt, Madrid and Paris.
Threlfall will lead a four-strong team of consultants specialising in media coaching and although the role is largely autonomous, it will be overseen by Whitney.
Threlfall comes to ECD after three years as co-presenter of CNBC's European Market Watch programme.
Working in New York and London, he has reported for Wall Street Journal Television, Bloomberg and Channel 4/ITN. He spent four years as anchor, reporter and producer for sister company Dow Jones's European and Asian Networks before joining CNBC in 2001.
ECD is keen to diversify away from financial companies and expand its client base into Plcs. The company already deals with Whitbread, Clariant and Emap, and it is hoped that Threlfall will shore up the existing client base while converting his raft of CNBC contacts into ECD clients.
Threlfall will be based in London while, in his new role, Whitney will divide his time between London and France.