The 14-month-old start-up expects to have 30 employees by the end of the year - more than doubling its size since January - after successfully seeing off competition from JWT, VCCP, Delaney Lund Knox Warren & Partners and DDB London, the incumbent on the business.
Thomson is an important addition to the agency's client list, which consists of Fiat, Alfa Romeo and the Unilever-owned Elmlea account, which it won last month.
Thomson, owned by TUI, spent £9.6 million on advertising last year, according to figures from Nielsen Media Research.
Agency Assessments International conducted the pitch because TUI wants to reposition the brand as a travel portal. It plans to advertise throughout the year, not just around peak holiday seasons.