Dylan Williams, a partner at Mother, sees his role expand, taking on responsibility for strategy globally and across Mother's group of companies.
The agency is also boosting its strategic offering by hiring James Broomfield from DDB, Cat Mathews from VCCP and Alice McGinn from Leagas Delaney as senior strategists.
Broomfield was the deputy head of digital strategy at DDB. He worked on the award-winning Monopoly "streets" campaign, the Virgin Media account, and the recently launched "Bud Ice Cold Index" campaign for Diageo Ireland.
He will be working on accounts including Match.com and Powerade at Mother.
Mathews was a senior strategist on COI and O2 at VCCP and worked on campaigns for the Home Office, Big Yellow Storage, the Prince's Trust and Sure. She will take over from Horner on Boots as the strategic lead.
McGinn has worked on Patek Philippe, InterContinental Hotels, Glenfiddich and Timberland at Leagas Delaney. At Mother, McGinn will be overseeing the No7 and other accounts.
Williams said: "Loz and Matt are the complete package, both as strategists and as people. Lateral minds, sharp eyes and big hearts. We've got a great team of strategists here. Loz and Matt will lead them brilliantly. We couldn't be happier."