Di Leo will leave FleishmanHillard Fishburn at the end of June to become a partner to iNHouse co-founder Jo Tanner from July.
The agency's other founder Katie Perrior left last summer to take up a role at Number 10, which she left last week. Perrior was obliged to give up her stake in the company on taking the role, so no longer has any formal link to the agency. Clients of iNHouse have included EY, Starbucks, News UK, Boris Johnson and Diageo.
Tanner said the hire of Di Leo would be "the first of several impressive appointments we will be making over the coming months".
Di Leo, who had joined Fleishman from Teneo in September 2015 to lead a new standalone, 12-strong public affairs team, said: "iNHouse Communications represents the best in super-smart, creative communications and I'm thrilled to be partnering with Jo to build on that reputation over the coming years.
"This is a hugely exciting time of change in British politics and together we'll be providing the kind of senior counsel that will help clients make the most of the opportunities that presents."
New roles at Fleishman
While Omnicom-owned Fleishman seeks a successor to Di Leo as head of public affairs UK, it has promoted Steph Bailey to the role of MD of its corporate practice in London.
Bailey has been at the agency for three years, prior to which she worked at Red and Edelman. She was made head of corporate at the same time as Di Leo's appointment was announced.
Fleishman has also taken on senior director and partner Michael Hartt from Burson-Marsteller. He will work in a cross-practice role, leading the firm's geo-political clients. Prior to working in London for Burson for five years, he worked for Edelman as a vice president in its international affairs practice in Washington DC.
FleishmanHillard Fishburn, the UK arm of the global agency, said its London corporate practice was Fleishman's largest globally.
FleishmanHillard Fishburn CEO Jim Donaldson said the corporate practice had seen "stunning success", driven by Bailey and now has 30 staff, and went on to say: "While we’re sad to see Michelle go and thank her for the brilliant contribution she has made to the business, we’re particularly pleased to have Michael Hartt join us next week to extend the exciting work we’re doing in the geo-political arena."
The division recently stopped working with PepsiCo UK.