During the German occupation, he wrote for German Lutheran pastors, Johannes Raum and Bruno Gutmann, who then used Mtui's notes for their own books on the Chagga people. During the British occupation Nathaniel was hired by Major Dundas, paying him 16 shillings for each full note book wrote about the Chaggan people. No one knows how many note books Mtui wrote for the 3 men, as many are missing, Gutmann preserved 9 of Mtui's note books. These 9 of Mtui's notebooks focus more on the histories of the central and eastern
Chaggaland. In 1911, Mtui started collecting ethnographic material for missionary Johannes Raum. His work was recommended by Johannes Schanz to Bruno Gutmann, who commissioned Mtui to collect oral historical traditions from southern and south-eastern Kilimanjaro between 1913 and 1919. Mtui documented these traditions in Kichagga, producing nine notebooks totaling approximately 1,000 handwritten pages. These notebooks were later used by Gutmann and Raum in their own publications, sometimes without proper attribution to Mtui. From 1913, Mtui served as a teacher at the German Lutheran mission in Ashira and Marangu and became the headman of the Mtui clan under Mangi Mlang’a in the same year. During the British occupation, Mtui was employed by Major Charles Dundas, the British administrator, to gather information about the Chagga past. Mtui visited various chiefdoms, conducting interviews and recording his findings in Swahili for Dundas. He was compensated for each notebook he produced, though the exact number of notebooks is unknown, as many are missing. Gutmann preserved nine of these notebooks, which provide significant insights into the history of the Marangu chiefdom and other regions in central and eastern Kilimanjaro. In 1924 Nathaniel Mtui together with
Joseph Merinyo founded The Kilimanjaro Native Planters Association (KNPA). It began as a cooperative association with the goal of buying and sharing spray equipment, but it swiftly developed into an organization that markets African coffee and serves as the political arm of the mountain's growers. The KNPA vigorously lobbied the
British run Moshi district office to defend their water rights, protect their coffee privileges, and give more land for the development of homesteads. They also denied the claims made by the settlers. Nathaniel Mtui died in 1927 at the age of 35. ==Personal life and death==