d'Assumpção joined the Weapons Research Establishment (now known as Defence Science and Technology Group) in 1958 and took up work pioneered by Alan Butement on Barra sonobuoys and later Kariwara towed arrays.[2] He worked his way up the organisation, culminating in the role of Chief Defence Scientist from 1987 to 1990. After leaving the Department of Defence, d'Assumpcao took up a role as professor at the University of South Australia and CEO of the Cooperative Research Centre for Sensor Signal and Information Processing until his retirement in 2000.[2]