The agency's UK managing director Jonathan Shore confirmed that Ellis had left to become a freelance consultant.
C&W has no firm plans to replace Ellis, effectively scrapping the role created just months ago to boost its international reach.
Shore said: 'One of his priorities was focusing on new markets. This is no longer high on the agenda and we think it's better to concentrate on our existing markets.'
Ellis's duties have been dispersed among senior managers across the company, he added.
Ellis took up the London based global role, which had a direct reporting line to C&W global president and CEO Steve Aiello, following a senior management reshuffle at the start of the year (PRWeek, 1 February).
Ellis had been with the agency for seven years. He joined as director of healthcare in 1995 and was promoted to healthcare global director three years later. In 1999 he was promoted further to become UK MD for the agency.
Shore, formerly deputy UK managing director, replaced Ellis when he took up the global role earlier this year.
Ellis was unavailable for comment as PRWeek went to press. He had previously told PRWeek, when he was appointed to the global role, that: 'It had become apparent that we needed to look at the network from a holistic international perspective and not just from a regional perspective.'
The agency has seven offices in Europe and six across North America but closed a number of offices last year as part of a restructure. Despite leaping six places to 12th spot in this year's PRWeek's Top 150 table of UK agencies, globally it has not performed so well.
In PRWeek's global rankings for this year the firm dropped one place to 19th with a drop of total global income of £3.7m.