Dunn became involved in the legal aspects for underwriting activities for companies being listed on the Montreal Stock Exchange and in 1902 he established the J.H. Dunn & Co. stock brokerage company. With $20,000.00 borrowed from his father-in-law, he acquired a seat on the stock exchange. Canadian business at the time was still heavily dependent on investment money from the British financial markets and Dunn became an expert at raising foreign capital for the underwriting of securities as well as selecting the right places to invest. Articulate, outspoken, and with a personality that commanded respect, Dunn had already made contacts with a number of influential people, including a number of railroad executives, one of the most powerful group of entrepreneurs of the day. As a stockbroker, Dunn's company put together a stock issue for William Van Horne, the former president of Canadian Pacific Railway. Van Horne had invested in the Cuba Railroad Company and wanted to raise capital for his "Havana Electric Company" venture. Already a close friend of fellow New Brunswickers, Max Aitken (the future Lord Beaverbrook) and a bright young lawyer named Richard Bennett, Dunn's brokerage work led to even more business connections with some of the elite of corporate Canada, including George Alexander Drummond, Henry Pellatt and the up-and-coming Izaak Walton Killam. As well, he would become a friend and admirer of the very successful American engineer-turned-entrepreneur, Frederick Stark Pearson. However, the London financial markets were the centre of the economic universe and encouraged by Pearson, in 1905 James Dunn made the decision to take up residency in London. There he ran a new merchant bank in partnership with the Swiss investment dealer, C.L. Fischer, all the while working in conjunction with his Montreal brokerage. At a time of rapid development in mechanized industry, as a result of large hydro-electric projects, shrewd investments, underwritings and stock promotions, James Dunn was soon a wealthy man. Dunn's brokerage house underwrote Pearson's ventures and sufficient capital was raised to allow Pearson to create a massive business empire that included the São Paulo Tramway, Light and Power Company in Brazil, the Mexico North Western Railway, the Mexican Tramway Company, and the Mexican Light and Power Company in Mexico, the British American Nickel Company in Canada and the Barcelona Traction, Light and Power Company. All was not easy and as a director of the Sovereign Bank in Canada that fell victim to