After graduating high school in Michigan, Joslyn enrolled at the University of California, Berkeley where he earned a B.S.
From 1928 to 1931, Joslyn worked in the food industry before returning to Berkeley as an instructor in the "Division of Fruit Products," then an administrative unit in the College of Agriculture.
Joslyn also wrote numerous books on fruit and vegetable juices and non-alcoholic beverages as well as food preservation by freezing and dehydration.
A charter member of the Institute of Food Technologists (IFT) when it was founded in 1939, Joslyn served as its President in 1965-66.
During World War II, Joslyn served in the United States Army Quartermaster Corps in Australia, New Zealand, and China, being discharged as a lieutenant-colonel.
For his military service, he earned the Legion of Merit, the Bronze Star, and a special order of Yun Hui with Ribbon from the National Government of the Republic of China.