He replaces Chris Harris, who left in 2010 after a year at the agency to join Cheil UK as its president and chief operating officer.
Hayhurst will be responsible for the day-to-day running of Leagas Delaney. He will work alongside the creative director, Nigel Roberts, who joined the agency last year, and report to the group chief executive, Margaret Johnson.
Johnson said: "There's a new generation of people such as James with blended experience who have a traditional agency background but with great digital and strategy experience putting that at the heart of what we do."
Hayhurst added: "The agency has seen great momentum in terms of new business recently - I hope to continue that success. We will also be focusing on developing creative and our integrated output."