Lewis had held the lucrative account, which is thought to be worth upwards of £300,000 a year, for almost four years.
Edelman will represent the HR and professional service provider in key jurisdictions across North America, Europe and Asia Pacific, with a specific focus on the UK and growing the firm's US business.
The wide-ranging brief encompasses corporate, consumer and b2b audiences. Edelman will help promote the brand to generate new business and raise the profile of the firm in specialist and national media titles.
It is understood that Edelman won the business after a final two-way battle against incumbent Lewis, overseen by Michael Custers, the US-based vice-president of global marketing at NorthgateArinso.
Nick Carlson, strategic marketing director at NorthgateArinso, praised Edelman's 'fantastic ideas' and 'understanding of the sensitive nature of our business'.
He said the firm's 'integrated service' and international reach were key factors in the appointment.
He added that an important aspect of the brief would be supporting growth in the US, where the company is looking to win market share from established names in the sector.
NorthgateArinso serves more than 50 per cent of FTSE 100 companies and 20 per cent of the Fortune Global 500 firms.
Edelman's involvement will be led by Alex Bigg, UK group MD, corporate & financial. The agency started work at the beginning of this month, during which time there is a handover period with Lewis.
NorthgateArinso specialises in offering technology to HR professionals. The firm has grown into one of the largest HR service providers in the world, with 31 offices globally, servicing companies in more than 70 countries.