Sky News has learnt that Scott Spirit, who left WPP three months after Sir Martin resigned in acrimonious circumstances last year, is to become chief growth officer at S4 Capital.
A statement confirming the appointment of Mr Spirit, who will also join the S4 board, is expected later on Wednesday.
His arrival will be announced alongside that of Elizabeth Buchanan, an advertising industry entrepreneur who also previously worked at WPP, who is joining S4 as a non-executive director.
One source said Sir Martin's plan to announce the appointments at the same time that WPP is holding its annual general meeting was "unlikely to be a coincidence".
The new recruits are not the first that Sir Martin has targeted from the empire he built over more than three decades at the helm.
Last year, he hired Michel de Rijk to run S4's operations in the Asia-Pacific region.
A string of acquisitions engineered by Sir Martin during the first year of S4's existence has seen it attain a market capitalisation of £600m, with further organic growth and takeovers potentially swelling that to beyond the £1bn mark within the next year or two, according to insiders.
Most recently, S4 struck deals to buy Caramel Pictures and ProgMedia, a programmatic advertising consultancy.
The purchases saw Caramel, whose clients include The Coca-Cola Company, Danone, Nestle and Unilever, become part of MediaMonks, S4's creative digital content producer.
Meanwhile, S4 is merging ProgMedia with MightyHive, the San Francisco-based advertising technology group which Sir Martin agreed to buy last December for $150m.
Since leaving WPP, Sir Martin has been a vocal critic of his former company's strategy, although Mark Read, his successor, has opted not to engage in a public battle with his former boss.
Mr Spirit's appointment by S4 comes almost a year after he left WPP to join Eureka AI, an artificial intelligence company.
His reunion with Sir Martin will come as little surprise to industry observers given the closeness of his working relationship with Sir Martin.
During a 15-year career at WPP, Mr Spirit held a number of roles including chief strategy and digital officer, and a seat on the board of Nairobi-listed WPP-Scangroup.
Sir Martin's return to the stock market, which came less than six months after his exit from WPP, was accompanied by an insistence that he was not seeking to compete with the company that he built over three decades into the world's biggest ad agency group.
Nevertheless, his takeover of MediaMonks nevertheless precipitated the launch of a legal battle in which WPP threatened to strip Sir Martin of share awards worth tens of millions of pounds.
That legal salvo from WPP exploded any semblance of a truce between the company behind giant advertising names such as J Walter Thompson and Ogilvy, and the man who became the longest-serving CEO in the FTSE 100.
The legal threat from WPP has since evaporated.
According to S4, ?digital media accounted for roughly 40% of the $500bn global advertising market, with that share expected to grow to approximately 55% by 2022.
While Sir Martin's pipeline of takeover candidates looks very different to those he pursued in the early part of his WPP reign, the playbook of bolt-on acquisitions and targeting of faster-growing geographies is similar.
Over more than three decades, he built WPP into a global force, presiding over hundreds of acquisitions in disciplines such as advertising, media buying, public relations and sports marketing.
Sir Martin remains a significant shareholder in WPP, with much of his wealth tied up in the stock of the company he took from a manufacturer of shopping baskets in 1985 to bestriding the global advertising industry.
By the time he stepped down in April 2018, WPP was valued by the stock market at more than £16bn, although a series of client losses and accelerating shifts in spending among global advertisers away from traditional media mean it is now capitalised at just £12.66bn.
Mr Read has embarked upon a drive to streamline WPP by merging some of its agency subsidiaries, while he is also exploring the sale of a controlling stake in Kantar, its market research business.
S4 declined to comment.