The aim of this study is to focus on the impacts and consequences of Illegal immigration that is one of the transnational threats that is seen to pose a challenge on the economy and the society respectively. This has affected Kenya‟s economy greatly as it has made a drift from other sectors that drive towards development into factors that are to promote and protect the welfare of the illegal immigrants that crossing borders into Kenya. To the society, that is, the host population there has been increased fear among them as they are not sure of the immigrants‟ agenda towards their lives, property, lands etc. This is seen to have an impact on Terrorism as some of these immigrants may cross the National borders with the intensions of overthrowing the government or promoting their interests first. The study has used the “push- pull” theory that stands out as one of the most appropriate approaches in the field of migration. The theory holds that migration is a phenomenon that is subject to the prevailing economic and political conditions in both the sending and the recipient countries. This is because the immigrants expect to be offered economic and political freedoms in terms of the opportunities they will be offered. Therefore, this research will be able to look at the reasons behind the presence of the illegal immigrants and their impacts on Kenya‟s National Security.