In the spring of 1919, Willi Hennig started school in Dürrhennersdorf, and subsequently was at school in Taubenheim an der Spree and Oppach. Rudolf Hennig described the family as calm; his father possessed an even temperament.
As of 1927, Willi Hennig continued his education at the Realgymnasium and boarding school in Klotzsche near Dresden. Here he met his first mentor M. Rost. Rost had an interest in insects and introduced Hennig to Wilhelm Meise, who worked as a scientist at the Dresdener Museum für Tierkunde (Zoological Museum Dresden). In 1930, Hennig skipped a year, and graduated on February 26 1932. As early as 1931, Willi Hennig composed an essay entitled Die Stellung der Systematik in der Zoologie ("The state of systematics in zoology") as part of his school work, published posthumously in 1978. It showed his interest as well as his considered treatment of systematic problems. Besides school, Hennig worked as a volunteer at the museum and, in collaboration with Meise, saw to the systematic and biogeographical investigation of the "flying" snakes of the genus Dendrophis that became his first published work.
From the summer semester of 1932 onwards, Hennig read zoology, botany and geology at the University of Leipzig. He would continue to visit the Museum in Dresden. There, he met the curator of the entomological collection, the Dipteran expert Fritz Isidor van Emden. Hennig saw him regularly until van Emden was expelled from National Socialist Germany for having a Jewish wife.
Hennig, however, developed a deep friendship with Emden's successor, Klaus Günther. Hennig was able to conclude his studies with a dissertation entitled, Beiträge zur Kenntnis des Kopulationsapparates der cyclorrhaphen Dipteren. By this time, Hennig had published eight scientific papers. Besides his 300-page revision of the Tylidae (now classed as Micropezidae), there were further papers on Diptera and the agamid genus Draco of gliding lizards. After his studies, Hennig was Volontär at the State Museum for Zoology in Dresden. On January 1 1937, he obtained a scholarship from the Deutsche Forschungsgemeinschaft (DFG) to work at the German Entomological Institute of the Kaiser-Wilhelm-Gesellschaft in Berlin-Dahlem. On May 13 1939, Hennig married his former fellow student Irma Wehnert. By 1945, they had three sons, Wolfgang (born 1941), Bernd (born 1943) and Gerd (born 1945).
During his time as prisoner of war, Hennig began to draft his most important contribution to systematics, not published until 1950. The rough draft was composed with pencil and ballpoint pen into an A4 exercise book, spanning 170 pages. During the war, he also published a further 25 scientific papers. Most of the correspondence and literature research was conducted by his wife, Irma.
He continued working at the German Entomological Institute in the Soviet Sector of Berlin, Berlin-Friedrichshagen, all the while living in the American sector in Berlin-Steglitz. On a trip to France with his son on August 13 1961, he heard of the impending Berlin Wall and returned to Berlin immediately to quit his appointment. Moving to East Berlin was out of the question, as Hennig held anti-communist views and already had a troubled relationship with the Sozialistische Einheitspartei Deutschlands (Socialist Unity Party of Germany, SED) that was the sole political party of East Germany, as Hennig had repeatedly helped employees of the institute gain employment in the West.
In West Berlin, Hennig was given an interim post at the Technische Universität Berlin as Distinguished Professor. He rejected offers from the U.S. Department of Agriculture in Washington, D.C. and an offer made by his friend Elmo Hardy, to become a Research Fellow at the University of Hawaii in Honolulu, citing as reasons that the education of his sons took priority for him, and that he needed to have the "cultural witnesses of the antique Greek-Roman Europe within ready access". He instead decided on a post at the State Museum for Natural Science in Stuttgart, where he was given a department for phylogenetic research. In April 1963, he moved to Ludwigsburg-Pflugfelden for this position. The scientific collections of the museum had been provisorily kept in Ludwigsburg and remained here until their re-housing at the new site of the museum at Stuttgart's Löwentor, in 1985.
Hennig's works in Stuttgart dealt almost exclusively with taxonomic revisions of Dipterans. For the Stuttgarter Beiträge zur Naturkunde, where he published the majority of his works, he completed 29 issues by the end of his life. Significant are the review articles published in Erwin Lindner's Flies of the Palaearctic Regions and the Handbuch der Zoologie. The cladistic methodology was also represented in several published works, foremost among them the article, Cladistic analysis or cladistic classification? A reply to Ernst Mayr (1974), intended as an internationally accessible reply to the criticism Ernst Mayr had made of Hennig's phylogenetic systematics.
Willi Hennig only visited international institutions abroad twice, in spite of receiving many invitations for guest lectures. From September 1 to November 30 1967, he worked at the Entomology Research Institute at Canada's Department of Agriculture in Ottawa and participated in the International Congress of Entomology in Canberra from August 22 to 30, 1972. With his wife, he also visited Bangkok, New Guinea (where much of Mayr's understanding of bird taxonomy originated) and Singapore on this latter trip. His stay in Canada was also used for visits to various entomological collections in museums of the US, including Cambridge, Chicago, Washington, D.C. and New York, always in the hope of finding further amber inclusions of Dipterans, that featured prominently in his research of the late 1960s and early 1970s. On the initiative of Klaus Günther, who by then held a chair at the Freie Universität Berlin, Hennig was given an honorary doctorate on December 4 1968; for health reasons, he could not accept this honour in person, and it was presented to him by Günther on March 21 1969 in Stuttgart. On the initiative of students whom he had lectured on several animal taxa, Hennig was made an honorary professor at the Eberhard Karls University of Tübingen on February 27 1970.
In the night of November 5 1976, Hennig died of a heart attack. He had previously repeatedly cancelled lectures with reference to his fading health, and had already had an attack on his journey to Ottawa. He was interred on November 10 at the Bergfriedhof in Tübingen.