In the last few decades, infrastructure has played a major role in supporting modern society. Moreover, there has been an increase in natural and human-induced disasters worldwide. In these situations, securing infrastructure is a major requirement. Confusion and misinformation can result if the boundaries of what constitutes critical infrastructure for a country are not clearly defined. Identification of critical infrastructure is the first step in the process of securing and protecting the available critical assets. This study aims to establish the infrastructure that can be classified as "critical infrastructure" in Sri Lanka. This includes establishing a clear margin for subsectors that fall within and operate within critical infrastructure and, consequently, ascertaining a clear definition for the critical infrastructure of the nation. This study adopted a mixed-method approach, which included an initial comprehensive literature analysis on infrastructure and the parameters involved in determining the criticality of infrastructure. Secondly, a questionnaire and semi-structured interviews were conducted to determine which infrastructure sectors would be most critical to Sri Lanka. The most significant infrastructures with the parameters of national security, economic sustainability, quality of life, public health, and safety, the criticality of infrastructure were ranked in both pre-and post-disaster scenarios, and an appropriate margin for the Sri Lankan critical infrastructure was demonstrated. The emergency services sector was found to have the most significant infrastructure in both pre-and post-disaster situations. Accordingly, the study reveals emergency services, water, energy, transportation, telecommunication, and finance as the critical infrastructures for Sri Lanka.