This study uses panel data for 3290 households to analyze the effect of an unconditional cash transfer on food and nutrition security among ultra-poor and vulnerable households. Study data are from an impact evaluation of the Government of Malawi's Social Cash Transfer Program, a cluster-randomized control trial that employs both random selection and random assignment. We use the difference-in-differences specification to estimate average treatment effects of the program on three components of food and nutrition security - current economic vulnerability, diet quantity, and diet quality. Results show protective program impacts during the lean season on diet quantity as beneficiary households are 11 percentage points more likely to consume multiple meals per day than control households (p < .001), have a higher level of apparent caloric availability (267.49 kcal, p < .05), are 10 percentage points less likely to be food-energy deficient (p < .05), and have a reduced hunger depth (111.11 kcal, p < .05). However, study findings indicate that after one year of program exposure beneficiary households have experienced little improvement in diet quality or current economic vulnerability to food insecurity. Clear policy and program implications emerge related to the purchasing power of the cash transfer, particularly during the lean season, and the importance of integrated social protection initiatives.