Ellis was appointed an assistant master at
Harrow School in 1952 and served as housemaster of the Headmaster's House there from 1961 to 1967. In September of that year he took up his first headship, at
Rossall School, and in 1972 transferred to
Marlborough College as Master (or head), In the
Michaelmas term of 1973, Ellis preached a sermon in the chapel of
Abingdon School, and in the summer of 1977 visited
Stowe School for the same purpose. Marlborough College was the first boys' public school in England to admit girls, and in March 1974 Ellis told
The New York Times that "The senior boys had a strong social need for the idea, and the girls clearly had a civilizing effect on them." While at Marlborough, Ellis was a member of the
Wiltshire County Council Education Committee (1975—1986). In 1983, he was elected as chairman of the Headmasters' Conference and retired from teaching in 1986. Soon after leaving Marlborough, Ellis was appointed by
Barclays Bank as its Graduate Recruitment Manager, a role he carried out until 1991. ==Voluntary work==