He married Katharine McMahon on August 31, 1876, in Washington, DC. They had a son, Owen McMahon Johnson (1878–1952), who became an American writer in his own right, and a daughter, Agnes McMahon Johnson (1880–1968). After their honeymoon that included attendance at the Philadelphia Exhibition, the couple relocated to the Murray Hill neighborhood in New York City. While in New York City, Johnson's love of nature and exploration extended to outings and ramblings. He was surrounded by social friends over for musical and literary evenings and consumed all forms of art, including opera and theater. His friends from the US and abroad included Tommaso Salvini, Paderewski, the Clemenses, Kipling and Eleonora Duse. By the 1890s, Johnson and his wife became friends with the inventor Nikola Tesla, for whom Johnson wrote a poem. He also collaborated with Tesla transliterating Serbian poems by Jovan Jovanović Zmaj in "The Century Magazine".