The first decades of the 21st century have witnessed special scholarly interest to a wide range of problems related to the socio-cultural study of collective mentality. This inevitably leads to the actualization of the research on mass historical sources that characterize the public mood in early 20th-century Russia. The “Letters of the Black Hundreds and Other Persons” are kept in the archival fund of the Union of Russian People in the State Archive of the Russian Federation. The fact that letters to political parties and organizations in the early 20th century, unlike to letters to Soviet authorities, have not been fully studied, determines the relevance of the article. The most effective way to analyze conservative and monarchist sentiments reflected in the “Letters of the Black Hundreds and Other Persons” is the method of content analysis, which allows getting the so-called hidden information from the source and significantly increasing its informative output. The result of applying this methodology to the “Letters of the Black Hundreds and Other Persons” was the identification of six significant semantic categories that are most often found in the epistolary texts and, thus, reflect a steady mood of their authors. An analysis of the semantic content of each of the categories made it possible to see all the currents of the public mood revealed by correspondents, and to identify the ideas of the right-wing monarchist ideology dearest to them. Paired occurrence of the most representative semantic categories allows supplementing their characterization. The textual analysis of the “Letters” demonstrated that through their messages the correspondents most actively broadcast their “support for the autocracy”, nationalist statements, anti-revolutionary sentiments, “support for and protection of the Orthodox faith,” upholding of the Union of Russian people and A.I. Dubrovin, personally. The fact that the expressions of support for autocracy and the Orthodox faith, nationalistic and anti-revolutionary statements are simultaneously found in most letters points to the existence of a stable conservative and monarchist mentality among the vast majority of correspondents, and the interpretation of these data in the article clarifies, in many aspects, historiographic assessments of their political and social views.