The Duke of Newcastle appointed a Private Secretary during his second term of office as First Lord of the Treasury (1757-62), and his 18th-century successors for the most part did likewise.

[1] At the same time the number of clerical staff working in the office was substantially increased, which eased the administrative burden.

The Principal Private Secretary was left to focus on arranging the Prime Minister's diary, offering advice and personal support, and overseeing certain particular areas (such as honours and appointments).

Since then the Principal Private Secretary has been considered a member of the permanent Civil Service, rather than a political or personal appointee, and as such may remain in office in spite of any change of administration.

When David Cameron became prime minister, he promoted his then principal private secretary to a new post of "Downing Street Permanent Secretary"; a position which took over as the top civil servant in the Prime Minister's Office and for the first time the head of the office held the highest rank in the UK's civil service.