He was also cultivating his interest in religious poetry and music, as in 1858 he wrote the lyrics for "I Love to Sing of Heaven" (customarily matched to tunes by Charles W. Dunbar or William B. Bradbury).

[4] In 1868, Hartsough, while in Wyoming but still in communication with Hillman, became musical editor of the Revivalist, a compendium of hymns and gospel songs which ultimately had 11 editions as items were removed or added.

[notes 2] There, during a revival meeting, Hartsough finalized, and soon published in the Revivalist, the one gospel song (both words and tune being his) for which he is remembered because of its international popularity—“I Am Coming, Lord” (also known by its first line “I hear thy welcome voice”).

In 1873 the song came to the attention of Ira D. Sankey as he sang for Dwight L. Moody during evangelistic campaigns in the United Kingdom, where it became known as "Gwahoddiad" from its translation into Welsh.

[6][notes 3] According to hymnologist William Jensen Reynolds, Hartsough during his ministerial career served 15 congregations and five Methodist districts.