Known familiarly as Molly, and professionally under her married name of Mary Proctor Cahill. She followed her father into the profession in 1935 after studying at Durham University School of Architecture. By 1938 she was architect to Alnwick Urban District Council. During the war she was a Civil Defence Ambulance Driver. Mary become a Fellow of the RIBA in 1946. Her work included hotels, houses and cottages for agricultural workers. At Swarland in the 1930s she laid out a Settlement for the Fountains Abbey Settlers Society where unemployed men from Tyneside were able to build homes for themselves and their families. She later worked with Lord Beveridge on the board of Newton Ayliffe new town.