Mary applied to work on a number of summer excavations. Wheeler was the first to accept her for a dig. It took place at St. Albans at the Roman site of Verulamium. Her next dig was at Hembury, a Neolithic site, under Dorothy Liddell, who trained her for four years until 1934. Her illustrations of tools for Liddell drew the attention of Gertrude Caton Thompson, and, in late 1932, she entered the field as an illustrator for Caton Thompson's book "The Desert Fayoum".