Kelly was chief inspector of inland letter carriers when he bought the rights to Critchett’s London directory in 1835, and was able to use his association with the Post Office to considerable advantage.
Kelly attracted widespread criticism for using Post Office employees to gather information for his London directory and was forced to abandon this practice in 1847. Nevertheless, by the 1850s his directories were pre-eminent in the south of England, and national dominance was assured once he took over the business of Pigot and Slater a few years afterwards.
Kelly’s success was based on the innovative content and formats of his publications. He championed the production of county and provincial directories covering all settlements in a given area. These provided a wealth of information about each place, often including a commercial, street and classified trades section. He also retained the title of Post Office Directory for some time, giving his publications an aura of officialdom.