Toumazis joined Yahoo in September 2014 as part of Yahoo's former vice president of EMEA Dawn Airey's attempt to boost the company's commercial offering.
He was responsible for business development and worked closely with Yahoo's global partnerships team.
In his more than 30 year career Toumazis has held many senior roles including chief commercial officer at Endemol Group, chief executive of the newspaper publisher Mecom and executive vice president and manager director of Disney-ABC-ESPN at The Walt Disney Company.
Earlier in his career Toumazis was in senior sales roles at ITV and managing director of advertising at Emap. He is the chairman of the board of Childline, the NSPCC helpline.
A spokeswoman said: "Tom Toumazis, Yahoo’s head of partnerships in EMEA, has decided to leave the company to explore external opportunities.
"We are extremely grateful to Tom for his contributions to our business and we wish him well."
Airey left Yahoo in September and has since joined Getty Images as chief executive.