Born in 1823, Joseph Leidy's scientific career was well underway by 1854. He had already provided dissection illustrations for Amos Binney's multivolume set (published in 1851), submitted several papers on snails to and become a member of the Boston Society of Natural History (1845), become a member of the Academy of Natural Sciences of Philadelphia (July of 1845 - the life membership fee was apparently paid for by Amos Binney), and been elected Chairman of the Board of Curators at The Academy of Natural Sciences (1846). He had published on paleontology (1845, 1853), been appointed as the Professor of Anatomy at the University of Pennsylvania (1853), and established himself as the leading authority on parasitology with the publication of "Flora and Fauna within Living Animals" (1853).

In 1858 Leidy named and described Hadrosaurus foulkii, the most complete dinosaur known skeleton of the mid-19th Century. Leidy's analysis demonstrated that the animal was bipedal, which directly counters the prevailing view of dinosaurs as established by Richard Owen, the world's leading authority.

In 1861 Leidy published "An Elementary Treatise on Human Anatomy" in response to his medical students' need for an anatomy textbook. The book established Leidy as the foremost anatomist in the United States.

Leidy's natural history research came to a halt in 1862, when Leidy's first Civil War duty began, serving as a pathologist at the Satterlee Military Hospital in Philadelphia. From this experience he contributed a series of reports on gastrointestinal disorders in a major government publication, "The Medical and Surgical History of the War of the Rebellion." In September, Leidy was appointed as chief surgeon for the City of Philadelphia and was charged with the responsibility of determining the fitness of draftees. He was also appointed as an inspector of the US Sanitary Commission. In addition, Leidy conducted a series of lectures on natural history at the University of Pennsylvania. Unlike his other lectures, these were open to the public. The program was so popular that it continued until 1867.

In 1863 Leidy became a founding member of the National Academy of Sciences. This society was immediately embroiled in controversy because of its exclusionary politics. Leidy joined the reform efforts, which ultimately succeeded in opening up the organization, but the experience must have reinforced his general dislike for professional societies.

Joseph Leidy married Anna Hardin in August, 1864. Relatively little is known about her, but accounts by friends and associates indicate the marriage was a happy one. Anna Leidy apparently had a serious interest in natural history. Joseph publicly credited her on several occasions for her assistance with his work. Moreover, Anna Leidy also joined the American Association for the Advancement of Science (AAAS).

In 1869 Leidy published "On the Extinct Mammalian Fauna of Dakota and Nebraska". This monograph is Leidy's definitive work on the White River Badlands fossils of South Dakota. It also covers fossils from the Niobrara River Valley of Nebraska. Shortly after, Leidy began to receive Eocene fossils from the Bridger Tertiary Formation of southwestern Wyoming.

In 1871 Leidy was appointed Professor of Natural History at Swarthmore College. He taught at Swarthmore until 1885 and retained emeritus status thereafter.

"Contributions to the Extinct Vertebrate Fauna of the Western Territories" was published in 1873. Much of this monograph is devoted to the Eocene Bridger fossils of Wyoming. At this time, the "Bone Wars" between O.C. Marsh and E. D. Cope escalated and Leidy significantly reduced his output in Vertebrate Paleontology.

In 1879 Leidy published "Fresh-Water Rhizopods of North America", the first significant publication on protozoology in the United States. He was also awarded the Medal of the Royal Microscopical Society from the distinguished British scientific society. This was the first of many prestigious awards from European and American scientific societies. Others include the Walker Prize from the Boston Society of Natural History (1880), the Lyell medal of the Geological Society of London (1884), an honorary degree from Harvard University (1886), and the Cuvier Medal from the Academy of Sciences of Paris (1888). In 1881, Leidy followed up by publishing "Parasites of the Termites", a study of endosymbionts in the intestines of a termite.

In 1884 the University of Pennsylvania created the Department of Biology and appointed Leidy as its director and as a Professor of Comparative Anatomy and Zoology. Then, in 1885 Leidy was appointed faculty president and head curator of the Wagner Free Institute of Science. During this time Leidy also wrote a chapter on human intestinal parasites in William Pepper's "A System of Practical Medicine. This represents the first comprehensive publication of human parasites in the United States.

Declining health and a demanding workload took their toll on Leidy. After several days of being bedridden, he died on April 30, 1891, apparently from a combination of strokes and arterial disease. He was 67 years old.