Stewart (pictured) joins the WPP-owned financial PR specialist as a partner.
He spent three-and-a-half years at Instinctif, where he focused on advising financial services businesses on their financial and corporate comms. Stewart's clients have included AJ Bell, Hastings, Shawbrook, Manolete and Arrow Global.
The qualified chartered accountant previously spent a decade at stockbroker Panmure Gordon, and was also chief financial officer at Swiss tech firm SandSIV as it prepared for its London Stock Exchange listing.
Buchanan chairman Richard Oldworth said: "He has a strong and proven track record in advising high-calibre clients and his expertise, particularly in the financial services and related industry sectors, will be of significant value-add to our existing and prospective clients."
Buchanan, which employs 41 people and generated an estimated UK revenue of £9.5m in 2018, has been expanding its services in recent years, hiring Weber Shandwick director Alex Brown in late 2017 to head its new corporate practice.
Last year it established a three-strong design team led by Kerrie Palmer, who previously headed the design specialism at Bell Pottinger.
Its existing financial services clients include Tadawul (the Saudi Stock Exchange), the Africa Finance Corporation, Cenkos Securities and Greensill Capital.