Leaders of the new agency, which will be called M&C Saatchi Talk, denied the merger was linked to the ongoing accounting scandal at the holding company.
M&C Saatchi Talk will have a near-90-strong workforce and annual revenue of about £8m.
Talk.Global chief executive Jane Boardman will be chief executive, with Chris Hides, global managing director and co-founder of M&C Saatchi Public Relations, made MD. Talk.Global chief operating officer Ryan Woor will have the same title at M&C Saatchi Talk.
Hides confirmed to PRWeek that Aldridge, who co-founded M&C Saatchi PR in 2012, is leaving the business.
The merger is to take place over the next three months and will be completed in April, when the new agency will move to a new, as-yet unidentified, office.
Asked about the reason for the merger, Boardman told PRWeek: "I've been thinking for quite a while that it's kind of crazy having two medium-sized agencies when the world – our PR world, specifically – is moving to the need for size, scale and stature.
"Also there's quite a lot of stuff we were struggling to fund, like a really expensive evaluation system, lots of central resources. Bringing the two companies together means we can really invest in that stuff. Clients these days are looking for such a range of services in the PR world and such a depth of intelligence, insight, data management, influencer, technology."
She added that growth has been "really difficult to achieve in this market, and this I think will really super-power our growth".
According to Boardman, there will be no redundancies as a result of the merger. The client lists of the two agencies are "really compatible", she said, with a lack of overlapping roles.
Hides added: "It's two good businesses coming together to make one great one. This is absolutely not a restructuring exercise. It's more about realising an opportunity to bring these two businesses together."
'We would have done this anyway'
The news comes amid the scandal that has engulfed the listed parent company M&C Saatchi following the multimillion-pound accounting error revealed last year, which led to plunging profits and share price, and the resignation of co-founder and executive director Lord Saatchi alongside three non-executive directors.
Boardman denied the merger was linked to the issues at the plc and said discussions about the move began in the summer before the crisis erupted.
"I definitely know we would have done this anyway, but it's very hard for the world out there to believe that, to be honest!"
She said most of M&C Saatchi's PR business had "not been affected" by the crisis, but admitted the constant focus on it was "a bit destabilising for some of the team".
"It hasn't impacted on us at all. We're still getting on with our business, we're still making the money we said we'd make, we're delivering our income," Boardman said.
Asked why the merger didn't include the group's other PR agency, M&C Saatchi Sport & Entertainment, Boardman said that is a "really different business" and the prospect of a merger with it "wasn't even vaguely mentioned or considered".
Speaking to PRWeek last September, after M&C Saatchi announced a £6.4m hit to the firm's financial performance in its interim results, chief executive David Kershaw was upbeat about the performance of the PR agencies.
"We’re pleased with the PR side, which is showing good business growth," he said at the time. "The numbers are good and reliable, and I think we’ll continue to see growth in the UK."
PRWeek has contacted Aldridge about her next steps.