Siebert started as a medical pathologist before leaving medicine to focus on linguistics. He studied medicine at the
medical school of the University of Pennsylvania graduating in 1938. Apart from his medical studies at Penn, he attended the linguistic talks and seminars given by
Franz Boas at Columbia and
Edward Sapir at
Yale, while working to attain his M.D. degree at Penn. He was further influenced by anthropologist
Frank Speck in his interest in Native American languages. Siebert explored vocabularies for flora and fauna, and also made arguments for
Algonquian people's place of origin. In 1969, he became a
Guggenheim fellow. In 1980, he received a grant from the
National Endowment for the Humanities for the creation of a Penobscot dictionary, a project that he had been working on since at least 1968.
Ives Goddard of the
Smithsonian Institution called Siebert "clearly the most brilliant and most competent avocational linguist working on Native American languages that there has ever been, hands down."
Karl Teeter, commenting on Siebert, called him "the dean of Algonquian linguistics". ==Personal life==