Jenner will join MediaCom Australia in April this year. Both agencies are part of Group M.
Tom George, the chief executive officer of MEC UK, said: "We're obviously sorry to see Toby go. As joint managing director, he's been instrumental in MEC's success, and he leaves at a time when the agency is in good health. We genuinely wish him all the best - he'll do a fantastic job at MediaCom Australia."
Jenner said: "It's been a fantastic four years, being part of some 200 people who have transformed MEC into one of the most respected media brands in the UK.
"However, the opportunity to work for MediaCom, the largest media agency in Australia, and the different challenges it presents was too good to pass up."
Jenner joined MEC in 2003 and was promoted to his existing role in late 2006. Before that, he was the managing director of Media Insight and also worked at Granada.