Brandis joined the British service in 1856 as superintendent of the teak forests of Pegu division in eastern Burma. During that time Burma's teak forests were controlled by militant Karen tribals. He introduced the "taungya" system (King KFS (1968). "Agro-silviculture (the taungya system)". University of Ibadan / Dept. of Forestry, Bulletin no. 1, 109pp.), in which Karen villagers provided labour for clearing, planting and weeding teak plantations. In return they were allowed to plant crops for the first few years between the trees. As the teak trees grew, villagers were moved to new land and the process was repeated. As a result of this process, many villagers became dependent on the state forestry service and local resistance to the state takeover of forests became increasingly difficult.
Brandis was initially interested in botany. His herbarium and botanical library which he shipped from Calcutta to Rangoon were lost when the boat carrying it capsized. This loss led him to shift his focus from botanical studies to forestry.
Brandis' work included determination of teak volume, rate of growth, identifying rate of harvest, developing forest protection plans against pests and fire. He also introduced timber purchase rules, clearing rules and the establishment of managed teak areas called conservancies with officers who were appointed as Conservators. After seven years in Burma, Brandis became Inspector General of Forests in India, a position he served in for 20 years. He formulated new forest legislations and helped establish research and training institutions. The Indian Forest College at Dehra Dun was founded by him. Brandis was created a Companion of the Indian Empire in 1878.
Brandis documented the sacred groves in Rajputana and Kans (woodlands) of Mysore, the Garo and Khasia hills which he visited in 1879, the Devarakadus of Coorg in 1868, and the hill ranges of the Salem district in the Madras Presidency in 1882, the Swami Shola on the Yelagiris, the sacred grove at Pudur on the Javadis and several sacred forests on the Shevaroys. He was among the earliest in India to formally link forest protection with local peoples.
Brandis was also involved in forestry education in England at Coopers' Hill and here he also influenced and mentored many like B. Ribbentrop, W. Schlich and C.A. Schenck of Germany, and Gifford Pinchot and Henry Graves (the first and second chiefs of the USDA Forest Service) of the United States. He influenced the forestry movement in the United States by mentoring Pinchot, Graves, and others who came to study with him in Germany, and through his voluminous correspondence with many other men such as Charles Sprague Sargent and Franklin Hough involved in establishing the U.S. national forest system. Pinchot relied heavily upon Brandis' advice for introducing professional forest management in the U.S. and on how to structure the Forest Service when Pinchot established it in 1905. His influence was so great that President Roosevelt, sent him a photograph in 1896 with the inscription "To Sir Dietrich Brandis, in high appreciation of his services to forestry in the United States. From Theodore Roosevelt."
He also took an interest in the forest flora of northwest and central India and Indian trees. Even after retirement Brandis continued to work on Indian forestry and at the age of 75 he started his principal botanical work, Indian trees, dealing with 4400 species. It was first published in 1906 and re-issued several times afterwards, the last time in 1971. He was posted at Balaghat in M.P as a principal of forester training institute for a long time in his service period.
Many species of plants are named after him: