Born August 4, 1886 near Wawa, Pennsylvania, just outside Philadelphia into a Quaker family, Francis Whittier Pennell grew up on his family’s farm. However, he was, according to himself, considered delicate and not expected to participate in active farm duties. He still developed an interest in nature at an early age and was encouraged by local amateur botanist Dr. William Trimble. He attended the Friends Westtown School in West Chester, PA. It was during this time that he was employed as a student assistant in the Herbarium of the Academy of Natural Sciences of Philadelphia. His education continued at the University of Pennsylvania, where he earned his B.S. in 1911 and his Ph.D. in 1913, he also completed a post-doctoral fellowship in the year after receiving his Ph.D. During his doctoral work, Pennell was encouraged by the head of the University of Pennsylvania’s botany department, Dr. John M. Macfarlane, to pursue the study of Scrophulariaceae for his thesis.

Following the completion of his post-doctoral studies, Pennell moved New York, where he served as associate curator of the New York Botanical Garden from 1914 to 1921. In 1921 he returned to Philadelphia and the Academy of Natural Sciences of Philadelphia as curator of plants.

Pennell was a firm believer in the necessity of personal field study and participated in many international collecting expeditions including to Columbia from 1917 to 1918 and 1922, Peru in 1925 and 1948, Chile in 1925, Mexico in 1934, 1935, 1937-1938 and 1940. He also spent considerable time collecting in the United States, particularly in the southeast and west. His dedication to field work was so strong that his 1948 expedition to Peru took place only a few months after suffering a heart attack.

Pennell dedicated his career to the study of Scrophulariaceae and was the foremost authority on the family. In 1935, he published his seminal work on the subject, The Scrophulariaceae of Eastern Temperate North America. At the time of his death, he was in the midst of producing a sister companion on the Scrophulariaceae of Western North America. He produced a significant body of scientific writing on plants outside Scrophulariaceae, botanical taxonomy, plant geography, floral evolution, and insect pollinators. He was also a frequent speaker on the same topics to clubs, schools, and other groups. He was particularly keen on the history of botany and botanists, particularly those from the Philadelphia area, of which he wrote extensively.

During his life he was a member of the American Association for the Advancement of Science, The Botanical Society of America, the Torrey Botanical Club, and the Pennsylvania Botanical Society. In 1942, he served as president of the American Society of Plant Taxonomists. In 1910 he joined the Philadelphia Botanical Club and served as president from 1942 until his death, and was editor of its journal, Bartonia. Pennell died of a heart attack following a Meeting of Friends in Media, PA on February 3, 1952. His wife, Anne, and his son, Francis C., survived him.