The FTSE 100-listed firm saw its reported revenue increase by 0.9% year-on-year to £3.59bn in the first quarter of 2019, but like-for-like revenue fell 1.3%.
The company – which last year saw its founder and chief executive Martin Sorrell depart under a cloud - said North America was its weakest performing region, with like-for-like sales there down 8.5%.
It pointed out that that while disappointing, the performance was in line with its budgets and actions it has taken since September with its creative and healthcare agencies, alongside leadership changes, are intended to address its US performance.
In the UK, WPP said like-for-like revenue was down 0.9%, which was a slight decline on 2018's performance.
Mark Read, WPP’s new chief executive – who took up his post in September 3018 - said: "As anticipated, our first quarter trading update reflects the impact of certain significant client losses in 2018, in particular in the United States.
“Although we face a challenging year, especially in the first half, I am encouraged by how well our people, agencies and clients are responding to our new strategic direction. Our expectations for the full year are unchanged."
In early morning trading, shares in WPP were down 0.7% at 899.40.