Grierson (pictured) joins from Talker Tailor Trouble Maker, which she joined in early 2019 as managing partner.
She is the first major hire under MSL London CEO Chris McCafferty, who arrived at the Publicis network agency in September last year.
MSL said: "Jo will work across the business to help accelerate growth in the agency and work closely with Publicis Groupe agencies such as Saatchi & Saatchi, Publicis.Poke and Publicis Sapient, delivering integrated, multi-channel work."
Warren has left after five-and-a-half years, having joined as head of digital, EMEA. He led MSL London on an interim basis for three months until McCafferty's arrival.
Donovan is set to depart at a later date, when the consumer practice will be led by her deputy, Simon Condon.
Grierson left Freuds at the end of 2018 after six years at the agency, where she ran the global Lexus, Toyota and McCain accounts. Prior to joining Freuds, she was a board director at Havas for six years.
McCafferty said: "I first offered Jo a job back in 2012. Eight years later, and we finally get to work together, growing the MSL business both as a standalone specialist and an integrated part of the Publicis Groupe. Bring it on."
In a statement, Grierson said: "Hope you’re ready."
In October, MSL announced that global chief executive Guillaume Herbette is stepping down. He isn't being replaced in the role, but instead local MSL offices are to report to Publicis leadership teams on a regional basis.
MSL London generated revenue of an estimated £37.5m in 2018, a rise of four per cent on the previous year, according to PRWeek’s UK Top 150 Consultancies table.