Joining in early October, Nicholas will be responsible for raising the organisation's profile and promoting its global humanitarian and development work.
She will take charge of the UK team, reporting to director of advocacy, communication and education Rudo Kwaramba.
Nicholas will work with a team that mobilises support for campaigns on access to education and trade justice, and that pressurises G8 leaders to achieve Millennium Development Goals.
Kwaramba said: ‘With a network of communicators, development and policy experts working and living in communities, we've a unique opportunity to bring news and opinion of global citizens to the UK public.'
PRWeek publishing director Peter Goldstein said: ‘We're very grateful for Kate's work over the years and will be announcing a succession plan shortly.'
The Christian agency has sought a top UK PRO since Jane Evans moved to the LA office in May.