Marrill is being succeeded by Nick Topham, former programme director for East Riding and Hull at Three Sixty’s sister company, Yorkshire Water.
Marrill said his departure from the company was “a personal decision” pertaining to travel issues.
Topham has more than a decade of experience within the wider Kelda Group in senior roles. His focus will be on ensuring that Three Sixty is ready for water market opening in April 2017, as well as “driving it towards achieving its aim of becoming a leading water retailer”.
Commenting on his appointment, Topham said: “I’m excited to be joining Three Sixty at a time that is seeing unprecedented changes in the water industry.
“Like my predecessor, I’m focussed on putting our customers at the heart of what we do and ensuring we make water work for them. We look forward to seeing how the market develops following deregulation.”
Kelda Group launched Three Sixty in September 2016, with the aim of “refreshing the water market”.
Marrill, formerly a director at Kelda Group’s non-regulated business Kelda Water Services, was appointed as managing director, having successfully developed the brand and proposition prior to launch.
In October, Yorkshire Water formally confirmed that it will transfer all of its non-household retail customers to Three Sixty.