The alarming rise of food insecurity in Nigeria necessitates prompt action and targeted research to avert nutritional deficiencies. This study assessed the nutritional deficiencies and increased susceptibility to negative health outcomes associated with food insecurity in vulnerable populations. The study employed a mixed-methods research design. The target population included households, healthcare providers, and policy stakeholders in the selected states. The sample size for the study was 500 which was selected using the multi-stage sampling procedure. Data collection instruments included structured questionnaires, semi-structured interview guides, and observational checklists. The questionnaire incorporated items from standardized instruments such as the Household Food Insecurity Access Scale (HFIAS) and the Food Insecurity Experience Scale (FIES). Content validity was established by panel of experts in public health, nutrition, and social research. The reliability coefficient was 0.89. The data collection process was carried out in three phases over six weeks. Data were collected through face-to-face interviews conducted at respondents’ homes. Quantitative data were analyzed with the aid of the Statistical Package for Social Sciences (SPSS) version 27, using means, standard deviations and logistic regression set at 0.05 level of significance. The result showed that 135(27.0%) agree that household member has been diagnosed with kwashiorkor or marasmus, and 151(30.2%) agreed that household member is suffering chronic disease (eg diabetes, hypertension). A very high relationship was found between access to nutritious food and monthly household income (r = 93) which contributed 87.7% of the variance in access to food. Household size also had a high relationship (r = 0.82) with access to nutritious food and contributed 68.8% of the variance in access to nutritious food. There was also a high relationship (r = 0.73) between occupation and access to food. Furthermore, both age (r = 0.56) and level of education (0.44) had a moderate relationship with access to nutritious food. There was a significant relationship between access to nutritious food and age [f(1,499) = 34.97, p = 0.00], monthly household income [f(1,499) = 2572.77, p = 0.00], household size [f(1,499) = 1100.312, p = 0.00], and area of residence [f(1,499) = 1348.82, p = 0.00]. It was concluded that, food insecurity plays a major and pervasive role in the reduction of nutritional deficiency and negative health outcomes among vulnerable groups in Nigeria, particularly children, the elderly, and rural and low-income urban dwellers.