After finishing secondary school John Kingsmill worked for a finance company and began studying accountancy. His studies were interrupted by war service, but he completed his course in 1948. His interest in theatre took him to London for a time, but he returned to Australia and his work as an accountant. In mid-life Kingsmill changed direction and took up work as an advertising copywriter. He retired in 1981.
In addition to other interests in painting and writing, Kingsmill co-founded a charitable organisation for the rehabilitation of the mentally ill. Kingsmill was also on the cast of the 1948 premiere of Sumner Locke Elliott's play Rusty Bugles q.v. (see his book My Brief Strut upon the Stage, 2001).
