After four-and-a-half years at the social media company, Sheil said he is "looking forward to a new challenge".
Sheil joined Twitter in 2012 as one of its first EMEA employees and was tasked with building out its first mobile team globally.
He tweeted yesterday, "[Joining Twitter] enabled me to build some of our largest customers globally, live and breathe an IPO, create & scale the first mobile apps organisation, build our go-to-market plan, launch all of our mobile products, travel the world (over 30 countries and 000's of flights!) and meet amazing brands."
Sheil added that he is looking to broaden his horizons in the mobile tech world.
Mobile advertising makes up 89% of Twitter's total ad revenue, the company reported last July,
Twitter's head of China, Kathy Chen, announced on New Year's Eve that she was leaving as managing director after just seven months.
Twitter was not immediately available to return a request for comment.