McCann began scuba diving in the early 1960s with Jacques Cousteau, exploring ancient Roman shipwrecks near Marseille. At the time, underwater archaeology was a new discipline and was "largely dominated by men." Between 1961 and 1962, she excavated the 7th-century Yassi Ada shipwreck with the National Geographic Society and University of Pennsylvania. While at the American Academy in Rome she expanded her Master's thesis into The Portraits of Septimius Severus, A.D. 193–211. In 2017, this was still "the major scholarly work on the portraiture of that emperor" according to her colleagues. Following her time in Rome, McCann taught at the University of Missouri from 1966 to 1971, and the University of California, Berkeley from 1971 to 1974. She was an active member of an international learned society that specializes in Roman pottery, which she became interested in as a result of her archaeological research underwater. In 1974, McCann joined the curatorial staff of the Metropolitan Museum of Art and led a lecture program related to archaeology. She published her research on Roman sculpture while at the museum in Roman Sarcophagi in the Metropolitan Museum of Art, which won the Outstanding Book Award from the Association of American University Presses and was recognized as an Outstanding Art Book by the Thomas J. Watson Library in 1978. McCann conducted excavations of Cosa between 1965 and 1987 that resulted in the 1987 collaborative work The Roman Port and Fishery of Cosa: A Center of Ancient Trade. This also received the Association of American University Presses' Outstanding Book Award, and the 1989 James R. Wiseman Book Award from the Archaeological Institute of America. A member of the Archaeological Institute of America's Board of Trustees, McCann founded its Committee for Underwater Archaeology in 1985. In 1989 she became the archaeological director of the JASON Project, collaborating with oceanographer Robert Ballard in surveying multiple shipwrecks of the Skerki Bank to inspire students within the project. This resulted in a publication in 1994 that is believed to be the first to detail archaeological research conducted in deep waters. McCann and Ballard discovered more shipwrecks when they returned to Skerki Bank in 1997. McCann was awarded the Archaeological Institute of America's Gold Medal Award in 1998 and presented with a Festschrift at the ceremony. She taught at Boston University from 1997 to 2001 and was a visiting scholar at Massachusetts Institute of Technology from 2001 to 2007.
Personal life
McCann married childhood friend Robert Dorsett Taggart in 1973. They lived in New York City but also spent time at their farm in Pawlet, Vermont. In 1985, McCann and Taggart established a lectureship in underwater archaeology. McCann presented her research through many venues—including a children's book that she contributed to and a general guide to some of her research—as a result of her "interest in the broad dissemination of archaeological information".