 In 1880 Dekum helped establish the Portland Savings Bank, of which he was named president in 1886, succeeding David P. Thompson. He helped organize the Commercial National Bank of Portland and served as its president. (The two banks occupied the same building and had overlapping directors, including John McCraken, George H. Williams, and Cyrus A. Dolph, as well as Dekum and Thompson.) He was at various times president of Columbia Investment Company, Oregon Land and Investment Company, Columbia Fire and Marine Insurance Company, Portland and Vancouver Railway Company, Trinidad Asphalt & Paving Company, and the Portland Exposition Company. In 1885, Dekum was one of 15 men named to the Portland Water Committee, empowered by the Oregon Legislative Assembly to acquire and operate a municipal water system for the city.
