Fergusson was introduced to engineering by her father, who made medical X-ray equipment. She later graduated from the University of Edinburgh with a BSc Hons in Civil Engineering in 1936.
Her career began as an unpaid trainee with Scottish firm Blyth and Blyth, where her talent was soon recognised. She quickly progressed through the company and was appointed as a senior partner – the first female to hold this position in a civil engineering company.
Fergusson contributed to many infrastructure projects in the Highlands and Islands, and gained a reputation for her endless energy and enthusiasm. She also worked on the Markinch paper mills and was responsible for the design of reinforced concrete bridges, steel-framed buildings and the River Leven water purification plant.
Fergusson was made a fellow of the Institution of Civil Engineers in 1957.
After retiring in 1978, she continued to work as a consultant but used her earnings to fund a university bursary for young engineers. She was awarded an OBE in 1979 and an Honorary Doctorate of Science at Heriot-Watt University six years later.