Denny soon turned from politics to business. He returned to being a partner with Horton and Phillips, this time by taking a half interest in Dexter Horton and Co., the bank founded by Horton and Phillips in 1870, which would eventually become Seattle-First National Bank. He was president of the Seattle and Walla Walla Railroad Company and an investor in the Great Western Iron and Steel Company. Later in life, he was active in the Society of Washington Pioneers and wrote a memoir, "Pioneer Days in Puget Sound".