After three years of teaching, he returned to the university to study medicine and biology, and after short stints of practice he became a researcher. His first job was as director of a nutrition institute in Aberdeen, which he started from scratch.
During the First World War he served as a military doctor for both the Royal Army and Navy, firstly in active duty and later as a researcher into military diets.
After the war, he spent another decade at the renamed Rowett Research Institute, for which he demonstrated a talent for raising considerable amounts of money allowing the institute to be considerably expanded. Through the 1920's, his own research was devoted mainly to animal nutrition, his focus changed to human nutrition both as a researcher and an active lobbyist and propagandist for improving people's diets.
After the Second World War, Boyd Orr resigned from the Rowett Institute, and took several posts, most notably at the FAO, where his comprehensive plans for improving food production and its equitable distribution failed to get the support of Britain and the US.
He then resigned from the FAO and became director of a number of companies and proved a canny investor in the stock market, making a considerable personal fortune, such that when he received the Nobel Prize in 1949 he was able to donate the entire amount to organizations devoted to world peace and a united world government.