Liz Taylor, the chief executive of the Association of Local Authority Risk Managers, has secured a new post as head of public risk services at Marsh.
Taylor has headed Alarm for more than a year and is due to step down in October.
She said she had enjoyed her time at Alarm, and added she was confident the association will continue its remarkable growth.
Alarm said it has yet to appoint her replacement.
Kevin McGlone, Alarm chairman, said: “We are sorry to see Liz go. She has done a great deal for us in terms of strengthening and improving the organisation, and raising our profile in the public and private sectors.”