Her appointment to the newly-created role comes at a pivotal time for WPP, which has reported two consecutive quarters of falling net sales and cut its forecasts three times this year.
Sir Martin Sorrell, the chief executive, has said he wants to simplify the group and get its agencies working more closely – a strategy he has dubbed "horizontality".
Pattison will keep her existing role as chief transformation officer of Group M, WPP's media-buying division, which she was given in May.
Her promotion makes her one of the most senior women within WPP, alongside the group chief counsel, Andrea Harris, and the global chief executive of J Walter Thompson, Tamara Ingram, and it signals the importance of Group M within the world’s biggest advertising group.
Pattison was previously global chief executive of Maxus but Group M is merging the agency with MEC to form Wavemaker after account losses at both agencies.
WPP has also merged other agencies, including putting Possible and Salmon into Wunderman and Neo@Ogilvy into Mindshare, in recent months as part of a move to simplify the group and cut costs.
WPP owns "more than 160 companies", according to the annual report, although it maintains it already has a relatively simple structure because it has a dozen or so key operating companies.
Sorrell said in a memo to staff: "Lindsay will oversee our 50 global client teams and help to deliver on our core strategic priority of horizontality.
"Our single most important task is to ensure we provide our clients with a seamlessly integrated offer, as they demand greater simplicity and efficiency from their marketing services partners.
"Horizontality will increasingly be reflected in the structure of the group itself, as well as our client teams, country managers and organic collaboration between our companies.
"Lindsay will play a key role in that project as we continue down the path of greater integration."
Pattison, 44, already represents WPP as a director on the board of Chime, the private equity-backed owner of VCCP, in which WPP has a minority stake.
She also launched Walk the Talk, an initiative to help senior women to advance in their careers, at Maxus and that programme is now being adopted across WPP.
WPP’s share price has fallen from £19 in March to close to £13 as sales have gone into reverse – a decline that Sorrell has blamed partly on FMCGs cutting spend.
When WPP reported a 1.1% drop in third-quarter net sales earlier this week, Sorrell suggested the ad business could be facing structural challenges in a commentary entitled "A changing industry?"