Ffitch was previously chief executive of Manning Gottlieb OMD and had little experience of the newspaper industry when he joined TMG in January 2016.
His future has looked uncertain after TMG recruited Nick Hugh from Yahoo to take on a newly-created position of chief operating officer and to drive "digital transformation".
Hugh, who started last month, said: "Robert and TMG have been exploring different roles for him in the commercial team.
"After much discussion, he decided that it was a good time for a change.
"We thank Robert for his contribution to the development of The Telegraph and wish him well for the future."
Hugh, who also has a background outside newspapers, plans to find a replacement for Ffitch.
Ffitch was responsible for a number of innovations during his tenure at TMG, including a major partnership with Apple News to be the exclusive reseller of general ad inventory on its news app in the UK.
He also introduced better online standards such as guaranteed ten-second views for advertisers before they had to pay for an ad.
However, like other newspaper publishers, the owner of the Daily Telegraph has faced a sharp decline in print ad revenues in the last 12 months as supermarkets moved spend to digital channels.
Group M estimates ad revenues fell 12% last year for UK national newspaper brands.
James Wildman, the incoming chief executive of Hearst UK and previously chief revenue officer of Trinity Mirror, said Ffitch had made a positive contribution to TMG.
TMG and Trinity Mirror have collaborated on a number of advertising projects, including for Audi.
Wildman said: "Robert is one of the media good guys. He is highly respected for his humanity and his integrity and his strategic approach to media."
Ffitch was a founding employee of Manning Gottlieb OMD in 1990 and rose to be its chief executive, winning Media Agency of the Year at the 2013 Media Week Awards. His decision to move to TMG was a surprise.
TMG remains one of Britain’s most profitable newspaper groups, making an operating profit of £51.7m in 2015.