Helmuth Heinrich Diedrich Holtz, artist, sailor, and Union soldier, was born on January 1, 1833, to Johann Christian Helmuth and Margaretha Christine (Berg) Holtz in Kappeln, Schleswig. The elder Holtz was a hoop maker there, and apparently his son learned his father's trade. Young Helmuth became a sailor, perhaps in Germany, and in 1860 visited the Texas ports of Indianola and Matagorda. From the bay on board the barque Texana he sketched views of the two towns and a view of the hotel at Matagorda. Holtz apparently sent these three views back to Hamburg, where they were reproduced by Eduard Lang's lithographic establishment.
On October 6, 1862, Holtz enlisted in the Union Navy. He gave his profession as "sailor." He served as coxswain on the steam-powered gunboat Westfield, the flagship of Commander W. B. Renshaw, commanding officer of the Texas blockade. During the Confederate recapture of Galveston on January 1, 1863, the Westfield ran aground and blew up prematurely while Renshaw and several others were still on board trying to scuttle her to prevent her capture. A Union transport apparently saved Holtz.
After seeing this action, Holtz was assigned to the USS Estrella, another light-draft, steam-powered gunboat, at Bayou Teche, Louisiana. Two of his drawings of naval actions in Louisiana were reproduced as engravings in the May 30, 1863, issue of Harper's Weekly. His final duty with the navy was quartermaster work on the store bark USS J. C. Kuhn, from which he was honorably discharged at New Orleans on October 10, 1863. He next worked as a draftsman in the Engineer Office, Military Division of the Gulf, where he helped produce maps for Gen. Philip H. Sheridan.
After the war Holtz remained in New Orleans, where, on May 2, 1865, he married Babetta (Strumpf), widow of Richard Beer. He was listed variously in the city directories from 1866 until 1870 as an engineer, civil engineer, and grocer. Around 1871 he apparently returned to Schleswig, where Babetta died in 1875. He was working there as a hoop maker again when he married his second wife, Kazia Auguste Nicoline Lorenzen, on August 6, 1880. By 1886 the couple returned to New Orleans, where Holtz worked as a draftsman. In the directories for the year 1887–88 he is listed as a leveler with the city surveyor's office. Several months before his death in New Orleans on August 23, 1915, he completed a pension application in which he listed three children.