Takhtajan worked at the Komarov Botanical Institute in Leningrad, where he developed his 1940 classification scheme for flowering plants, which emphasized phylogenetic relationships between plants. His system did not become known to botanists in the West until after 1950, and in the late 1950s he began a correspondence and collaboration with the prominent American botanist Arthur Cronquist, whose plant classification scheme was heavily influenced by his collaboration with Takhtajan and other botanists at Komarov.
The "Takhtajan system" of flowering plant classification treats flowering plants as a division (phylum), Magnoliophyta, with two classes, Magnoliopsida (dicots) and Liliopsida (monocots). These two classes are subdivided into subclasses, and then superorders, orders, and families. The Takhtajan system is similar to the Cronquist system, but with somewhat greater complexity at the higher levels. He favors smaller orders and families, to allow character and evolutionary relationships to be more easily grasped. The Takhtajan classification system remains influential; it is used, for example, by the Montréal Botanical Garden.
Takhtajan also developed a system of floristic regions.
Takhtajan is a member of the Russian Academy of Sciences, as well as a foreign associate of the U.S. National Academy of Sciences since 1971. He is also the academician of the Academy of Sciences of the Armenian SSR, the president of the Soviet All-Union Botanical Society (1973) and the International Association for Plant Taxonomy (1975), member of the Finnish Academy of Science and Literature (1971), the German Academy of Naturalists "Leopoldina" (1972) and other scientific societies.
In 1932 he graduated from the Soviet (All-Union) Institute of Subtropical Crops (Tbilisi). In 1938-48 he headed a Department at the Yerevan State University, in 1944-48- director of the Botanical Institute of the Academy of Sciences of the Armenian SSR, Professor of the Leningrad State University. His an author of works on the origin of flowering and paleobotanics. He developed a system of higher plants. He worked on the "Flora of Armenia" (vol. 1-6, 1954-73) and "Fossil flowering plants of the USSR "(v. 1, 1974) books.