After college, he joined the Maine state as an entomologist. From 1862 to 1865 he was enlisted into the First Regiment of Maine Volunteers as an assistant surgeon. During the marches he collected insects. His chief work was the classification and anatomy of
arthropods, and contributions to economic
entomology,
zoogeography, and the
phylogeny and metamorphoses of
insects. At Boston he became a librarian and custodian for the Boston Society of Natural History working also on his Labrador collections. In 1866 he went to the Peabody Academy of Science along with Putnam, Morse and Hyatt. He left in 1878 to accept a position of professor of Zoology and Geology at Brown University. Packard was appointed to the
United States Entomological Commission in 1877 where he served with
Charles Valentine Riley and
Cyrus Thomas. He wrote school textbooks, such as
Zoölogy for High Schools and Colleges (eleventh edition, 1904). His
Monograph of the Bombycine Moths of North America was published in three parts (1895, 1905, 1915, edited by T. D. A. Cockerell). He was elected as a member to the
American Philosophical Society in 1878. ==Personal life==