Howley has been handed the new role of chief operating officer of Publicis Media UK, whose agency practices, including Content and Data Sciences, will report to him. Publicis Media agencies include Zenith, Starcom, Blue 449, Digitas, Performics and Spark Foundry.
His tasks will include creating greater collaboration between the agencies, as well as driving the overall business performance of Publicis Media.
Howley's promotion frees up the role of Zenith chief executive, which Cummins will step into from her current role of UK group managing director of the agency.
Publicis Media UK, which Frogley took charge of in October last year, is in planning for moving all its parts under one roof in White City in the second quarter of 2019.
Frogley praised Howley’s "superb job at Zenith UK with a retention and organic growth rate record that is second to none" and described Cummins as "well-respected by clients and media owners alike".
Howley, who took over from Grant Millar as Zenith chief executive in 2015, said he hoped to "contribute significantly in building out [Publicis Media’s] model quickly and at scale".
Cummins, who also took over Howley’s former role in 2015, said she was "chuffed to bits" about leading Zenith and hoped to continue Howley’s "great work".