Kelly fills the empty slot left when Rachel Bremer joined Twitter as its European communications manager in June.
She will be responsible for UK and European operations and growing the company's client base in Europe.
Reporting to Spark’s CEO Alan Soucy, who is based in the office in California, Kelly will look to build on the agency’s consumer, tech and digital clients.
Current clients include Inq Mobile, Index Ventures, Moo.com, Techlightenment, Good Technology and Etsy.com.
Kelly said: ‘I’ve worked in digital for the past 15 years and I’m looking forward to building on the clients we already have here.
‘It's a thrill to be working with companies who are changing the way we live.’
She added: 'I'm also looking to expand the team here.'
Before Ask Kelly was head of PR at Freeserve and at Yell.com and worked for PR agencies Shandwick Scotland and Barclay Stratton.