The business was held by OMD UK, but AMS won the account ahead of a campaign to promote the brand that will run later this month.
Fallon, the creative agency that developed Innocent's "made by nature" positioning, has created the new work, which will initially involve outdoor and press.
Although Innocent spent less than £1 million on advertising over the past year, it is expected to increase its budget as it grows. Reports have suggested its spend could be as much as £2.5 million.
Fallon has worked with Innocent on an Innocent summer initiative involving press ads continuing the "made by nature" theme. Ads have only previously run in London, Leeds and Glasgow but will appear nationwide.
Richard Reed, the marketing director at Innocent, said: "2004 is all about building on the great double-digit boost in brand awareness created by last year's outdoor and print campaign. The guys at AMS have got to grips with the challenge of identifying who our consumers are and how to reach them this time around."