After leaving the military, he traveled to New York City to join his elder brother William Cunningham, arriving in 1903. Denis followed his brother Williams example and went into the antique business, working at The Antique Shop on Fifth Avenue and A.J. Crawfords also on Fifth. Denis met his future wife Ada in New York City. Ada was from a long established family, the Ketchums of Buffalo New York who helped establish the Buffalo and Erie area of New York before the Revolutionary War. Denis and Ada Charlton Brennan wed 14 September 1907 in New York City; the couple would have three children, daughter Muriel Denise (born August 1908), son Kearns (born August 1909) who died within hours of his birth and lastly, daughter Alice Eleanor (born December 1910). Ada and son Kearns are buried along with several generations of Ada's Brennan in-laws, in the Woodlawn Cemetery in the Bronx, New York.