In 1955 Hartley graduated in botany with the academic degree Bachelor of Science at the University of Wisconsin-Eau Claire. In 1957 he received his Master of Science and in 1962 his Ph.D. degree at the University of Iowa.
From 1961 to 1965 he led an expedition of the Commonwealth Scientific and Industrial Research Organisation to New Guinea for the study of phytochemicals. From 1965 to 1971 he was associative curator at the Arnold Arboretum of Harvard University in Cambridge, Massachusetts. In 1971 he became a Senior Research Scientist at CSIRO Plant Industry, Canberra, Australia.
Thomas Gordon Hartley became notable for his study on the family Rutaceae. He described several new plant taxa and genera from Papua New Guinea, New Caledonia, Australia, Peninsular Malaysia like Maclurodendron and Neoschmidia and wrote revisions on genera like Zanthoxylum and Acronychia. In 1989, he and Benjamin Clemens Stone made a major revision of the genera Melicope and Pelea when they largely synonymized the genus Pelea with the genus Melicope.
- 1966: A revision of the Malesian species of Zanthoxylum (Rutaceae)
- 1967: A revision of the genus Lunasia (Rutaceae)
- 1969: A revision of the genus Flindersia (Rutaceae)
- 1970: Additional notes on the Malesian species of Zanthoxylum (Rutaceae)
- 1974: A revision of the genus Acronychia (Rutaceae)
- 1975: Additional notes on the genus Flindersia (Rutaceae)
- 1975: A new species of Zanthoxylum (Rutaceae) from New Guinea
- 1977: A revision of the genus Acradenia (Rutaceae)
- 1977: A revision of the genus Bosistoa (Rutaceae)
- 1989: (with Benjamin Clemens Stone): Reduction of Pelea with new combinations in Melicope (Rutaceae). Taxon 38: 119–23
- 2001: Allertonia. On the taxonomy and biogeography of Euodia and Melicope (Rutaceae).
- "Hartley, Thomas Gordon (1931 - 2016)". Council of Heads of Australasian Herbaria Australian National Herbarium.
- American Men of Science: Physical and biological sciences. Bowker. 1965. p. 287.
HARTLEY, DR. THOMAS GORDON, b. Beaumont, Tex, Jan. 9, 31; m. 53; c. 7. SYSTEMATIC BOTANY. B.S, Wis. State Col. Eau Claire, 55; Ph.D.(bot), Iowa, 61. Instr. blol, Wis. State Col. Whitewater, 57-59; res. scientist, phytochem. surv. of New .
- IPNI. T.G.Hartley.
- Short biography from the Nationaal Herbarium Nederland
- "Thomas Gordon Hartley". International Plant Names Index (IPNI). Royal Botanic Gardens, Kew. Retrieved 26 March 2008.