James joined Adconion as UK managing director in early 2006 and helped build its operations from scratch. He was instrumental in helping the firm increase its UK market presence.
It is unclear why James has left, but a spokesman for Adconion said he will be "replaced in due course".
In September 2008, James appointed Mark Connolly, former head of commercial development at Yahoo, to commercial director, to assist him in leading the business and its 30-strong team.
For the time being, all of James' responsibilities will be dealt with by Connolly, vice-president, network operations Veerle de Lombaerde, global marketing director Joanne Cox and president of Adconion's European operations Arndt Groth.
Last month, Adconion, one of the world's largest independent online audience networks, created the first online video campaign for The Thai Childrens Trust.
The campaign is aimed at women aged 30-plus and encourages them to help the Thai Children's Trust raise funds for refugees in Thailand, among the neediest children in south-east Asia.