Harri started as the new mayor's director of communications this Monday after the two men reportedly sealed the deal in a 90-minute chat in Johnson's back garden last Sunday.
Harri told PRWeek: 'I am keen to stress I have been extremely happy at Fleishman-Hillard. I feel embarrassed that my time here was so short and will miss my new friends and colleagues enormously. They, and the managers here are terrific, brilliant at what they do and wonderful people to spend time with.'
He added: 'As you can imagine, jobs like this don't come up very often, and, though horribly bad timing as far as my career at Fleishman-Hillard was concerned, the prospect of helping Boris Johnson to deliver his vision for London was too fascinating to turn down.'
PRWeek previously revealed that Harri had been in the frame to be the Conservative Party's director of communications, before Andy Coulson took the role.
One Fleishman-Hillard insider said the agency's bosses had always expected that Harri might eventually take a job with the Tories, but there was 'frustration' that he had left so quickly. However, F-H's UK public affairs director Nick Williams said: 'I would like to thank Guto... and wish him all the very best in his new role.'