Brine is leaving to 'try something different', according to a spokesman, who added that his departure was amicable and that he will remain a shareholder in the company. He will also continue in his role of chairman of the Marketing Communication Consultants Association.
Glasson was previously chief operating officer at Gyro, a role he has held since 2002. He has been in charge of eight acquisitions over the past two years as well as opening offices in Dubai and Madrid.
He said: "In January this year we secured new investment of £55m, and we have some exciting new opportunities to continue Gyro's expansion in the UK and overseas. 2008 will be yet another record year for the company, and we head into 2009 full of optimism."
The chief operating officer role is being taken over by Richard Perry, who has been with Gyro since 1995. He is promoted from the role of vice-president of the EMEA region and managing director of the UK and Ireland business.
Brine said: "I wish Richard Glasson every success for the future."
Gyro's clients include Sony, American Express and Adobe and its regional offices include London, New York, Dublin and Paris.