Graham will work across The Assembly's clients, including auditor Ernst & Young, beer manufacturer InBev, and book publisher Penguin, and will take an active role in developing new business.
He will report to The Assembly's founders Trevor Hardy and Steve Dunn, and he will work alongside former Ogilvy UK vice-chairman Larissa Joy, who joined the agency as a partner this autumn.
Prior to joining SapientNitro in January 2010, Graham worked in account management at agencies such as Saatchi & Saatchi, Publicis, TBWA London and Lowe Lintas.
He started his career at Faulds Advertising in 1995, where he worked as an account handler until 2000.
Joy started working with The Assembly as a consultant earlier this year. Her varied career includes roles such as chief operating officer of PR firm Weber Shandwick and global chief operating officer of private equity firm Actis LLP.
The Assembly has bumped up its team with a number of appointments in recent months, including business director Muna Nageh, account director Sophie Purnell and producer Anneliese Wensley.
Trevor Hardy said: "We are thrilled to have James join our other recent recruits at such an exciting time for the business. His expertise will be a fantastic asset to the agency, supporting our growth and new clients."
Follow Maisie McCabe on Twitter @MaisieMcCabe