While the theory of representative bureaucracy originates from concerns about the class composition of the public sector workforce, questions of class background have been notably absent in subsequent scholarship. In this paper I take advantage of new data on the class backgrounds of UK civil servants (N= 308, 566) to, first, explore descriptively how class shapes the composition of the civil service, both vertically in terms of occupational grade and horizontally in terms of department, location and profession. I show that those from working-class backgrounds are not only under-represented in the Civil Service as a whole, but this skew is particularly acute in propulsive departments like the Treasury, locations like London and in the Senior Civil Service. This initial descriptive analysis then acts as the staging point for the central qualitative component of my analysis, drawing on 104 in-depth interviews across four case-study departments. Here I identify three unwritten rules of career progression that tend to act as barriers for those from working-class backgrounds; access to accelerator jobs, organisational ambiguity in promotion processes; and sorting into operational (versus policy) tracks that have progression bottlenecks. This analysis highlights the need for more work on class representation, as well as underlining how representative bureaucracy may be impeded by patterns of horizontal as well as vertical segregation, particularly in work areas that have an outsized influence on policy design.