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.