The spatial expression of urban centrality in big cities is generally associated with several urban nodes. The major purpose of this article is to identify the spatial pattern of urban sub‐centres in Mexico City during the period 1989–2009 and how changes in urban centrality have given way to a more polycentric urban structure. A double‐threshold method was used for comparing the polycentric structure of a single city over time examining the variables of number of jobs and tertiary employment density; this approach for identifying employment centres is proven in a megacity of a developing country.