Forde’s appointment comes weeks after she left rival Publicis Groupe, where she spent 20 years and rose to be chief executive of Spark Foundry.
She is Interpublic’s third, significant external hire in London in the last 18 months after Caroline Foster Kenny joined as EMEA chief executive of IPG Mediabrands, Interpublic’s media division, and Richard Morris became UK chief executive of Initiative.
"We’re making investments right across IPG Mediabrands and a big part of that is talent and people," Foster Kenny said.
"We are upweighting not just with tools, capabilities and products but also leadership.
"The UK needs to become much stronger – that’s part of my remit."
Forde, who has also worked at Zenith and was managing director of P&G United, Publicis Groupe’s agency for Procter & Gamble, joins in July.
Foster Kenny said Forde is "so intuitive with clients", has "an entrepreneurial spirit" and "brings a special energy that aligns with UM’s business ethos".
Forde will fill a gap left by Russell Place, who quietly departed as managing director earlier this year to spend more time with his family.
Foster Kenny pointed out UM has had "a consistent and stable" management team but she felt "now is the time to bring in fresh energy".
Forde said UM felt "like a natural fit for me", adding: "I’ve always enjoyed working with disruptive businesses and UM certainly promises to play to this agenda."
UM has 197 staff, up 16% on a year earlier, and estimated billings of £191m, according to its Campaign School Report, which was published this week.
UM’s clients include Spotify and Accenture.
It is the 13th largest media agency in the UK, directly behind the largest independent shop, the7stars, in 12th spot but ahead of Initiative in 15th, on the basis of Nielsen estimates.
IPG Mediabrands is ranked fifth by holding company with about a 4% share of the UK media agency market, behind WPP’s Group M, which has more than a third of billings, and Dentsu Aegis Network, Omnicom Media Group and Publicis Media with about 16% each.
Although Foster Kenny wants IPG Mediabrands to grow in the UK, she said the ambition was "to be the best" – "not the biggest".
She described billings for paid media as a "very blunt, out-dated instrument to measure success", noting 60% of IPG Mediabrands’ revenues come from owned and earned media services.
"It’s about being bigger and stronger and coming up with a model that’s different for clients," she said, adding "our size is our advantage right now because we can move at speed".
Bigger rivals, notably WPP, which has just lost its chief executive, Sir Martin Sorrell, have admitted they need to simplify their operations because of their size.
Foster Kenny said the rise of programmatic media, which is now over 50% of UK digital ad-buying, meant that digital smarts matter more than scale.
"The relevance of scale is being reduced, it’s becoming less relevant," she said. "We talk about being ‘big enough’."