David Penhallow was born in Maine, and received his B.A. from Boston University in 1873. He became research assistant to William Smith Clark, President and Professor of botany and horticulture at Massachusetts Agricultural College. He followed Clark to Japan when the latter was appointed President of Sapporo Agricultural College in 1876 and taught there for four years. In 1880, Penhallow returned to the United States, where he conducted agricultural research at Houghton Farm, New York. He came to McGill as Lecturer in botany in 1883, and became Professor of botany in 1885. In 1901, he was named Macdonald Professor of Botany. An authority on palaeobotany, Penhallow wrote a Manual of North American Gymnosperms and A Review of Canadian Botany, as well as nearly two hundred scientific papers. He became senior curator of the Redpath Museum in 1908.