Three UK chief marketing officer Shadi Halliwell is leaving the business as part of a major restructure that will involve the merging of its marketing and customer teams.
Halliwell, who joined in 2017 and is a Campaign Power 100 marketer, will depart in the summer after overseeing the delivery of its latest 5G marketing campaign.
Elaine Carey, currently chief commercial officer at Three Ireland, will move to the same role in the UK later this month to lead the newly combined customer and marketing team.
Three said the changes were being made to "maximise the 5G opportunity and fully leverage our investment in our IT and network".
Chief executive Dave Dyson is also stepping down from his role after nine years for personal reasons, but will continue to serve on the board. He will be replaced by Three Ireland chief executive Robert Finnegan, who will take on both roles.
"Since Shadi joined us in 2017, she has made a huge impact on our brand, leading our successful ‘#PhonesAreGood’ advertising campaign and helping us to build partnerships with household names like easyJet and Snapchat," Dyson said.
"More recently Shadi led the signing of our exciting new sponsorship deal with Chelsea FC and our 5G campaign, which highlighted our leading 5G spectrum holding and showcased our ultra-fast capabilities through an immersive audience experience at London Fashion Week.
"On behalf of Three, I would like to thank Shadi for the enormous contribution she has made and we wish her all the best for the future."
Halliwell said it had been a "privilege" to work for the company.
She added: "Three has always been a pioneer in the telecommunications industry and I am confident that its bold approach to marketing and customer engagement will continue to grow from strength to strength in the years ahead."