Locke, one of the best-known ad buyers in London’s media industry, turned 60 last September but his departure has still come as a surprise.
He is known as a savvy operator who has previously been described as "gobby, funny and smart" with a "hold-no-prisoners style of negotiation" by Campaign.
As managing director of Publicis Media Exchange, he oversaw over £1bn in annual media spend in the UK for the world's third-biggest ad group.
Last year, he slashed Starcom’s media spend with the Daily Mail during a four-month stand-off over trading.
His street-wise trading abilities ensured his longevity in a front-line role in an industry where few buyers survive past the age of 50.
Publicis Media, which has not yet decided on how it might replace Locke, said in a statement: "Chris Locke has decided to leave his role as managing director of PMX in the UK, Publicis Media’s trading arm, to pursue other interests.
"We thank Chris for his contribution to our business over the years and wish him well in his future endeavours."
Publicis Media declined to comment further. Locke could not be reached for comment.
Locke, who has been with the company for 27 years, joined the media department of DMB&B, which became The Media Centre and then MediaVest in 1999. MediaVest merged with Starcom in 2001 and Publicis Groupe then bought the group in 2003.
He was put in charge of Vivaki, the UK trading arm, in 2008. Vivaki was rebranded as Publicis Media Exchange in 2016.
Locke featured regularly in the Campaign Annual’s list of the top ten media buyers.
He liked to joke: "I wouldn’t say I was the best buyer in the UK, but I’m in the top one."
Locke was dubbed the "Peter Pan of media" by his Publicis colleagues in a 60th birthday announcement for him that appeared in the Daily Telegraph last year.