Robert Rusk was born in Ayrshire and after service as a pupil teacher attended the Glasgow Free Church Training College from 1898 to 1901. Concurrently with his Training College course he attended classes at Glasgow University, graduating with 1st class honours in mental philosophy in 1903. He then pursued further studies at the University of Jena, where he graduated PhD in philosophy in 1906. His thesis was written in German. German scholars were pre-eminent in psychology and psychiatry at that time. Rusk achieved a BA at Cambridge University in 1910.
After brief service with the St Andrews Provincial Committee for the Training of Teachers, he transferred to Jordanhill College from 1923 to 1946, rising to Principal Lecturer in Education. He also taught the EdB (later the MEd) course at Glasgow University and was heavily involved with the Scottish Council for Research in Education (SCRE), serving as its first director, from 1928 to 1958. On SCRE there was great rivalry between Rusk and William Boyd, Lecturer in Education at Glasgow University, whom Rusk had displaced as Lecturer on Education at Jordanhill. Rusk represented the cognitive stream of educational research, particularly intelligence and mental testing, whereas Boyd was more interested in special educational needs and felt that SCRE was focusing too narrowly on mental testing. Rusk won that debate.
Rusk produced a number of histories of education and lives of the great educators, which ran into many editions. He also researched and published the first history of teacher training in Scotland, and delivered a public lecture illustrated by lantern slides to mark the centenary of teacher training in 1928. He was meticulous in his use of printed sources and oral testimony from those who had known the early pioneers, and file copies of his books were interleaved with notes and press cuttings to update the text for the next edition.