Improving nutrition-sensitive food security is important in low-income, food-deficit nations like Lao PDR, where subsistence farmers rely on a single rainfed lowland rice crop in the wet season. Access to resource conservation technologies, small on-farm water storages and electricity are starting to allow farmers to consider post-rice crops. With water availability still limited, short-duration pulse crops may be viable. This paper examined grain yield and water use during grain filling of eight short-duration mungbean genotypes after rainfed lowland rice at Champhone and Pakse in southern Lao PDR in the 2013 dry season. Two supplementary watering regimes were used, watered every 15 days (at 15, 30 and 45 days), and watered as needed at the first sign of visible wilting (at 23 and 45 days), with three replicates. Soil volumetric water content was measured by time-domain reflectometer. Over two drying cycles, VC1913A, VC3890A and VC7118A used significantly less water than other genotypes at both sites. Genotypes VC7118A and NM94 were higher yielding at Champhone, and VC6310 was higher yielding at Pakse. The relationship between grain yield and water use during grain filling was less straight forward, as it applied within apparent water-use efficiency categories. Overall, VC7118A was superior, due to its effective combination of higher apparent water-use efficiency during grain filling, lower water use in grain filling, and higher grain yield. NM94 also performed well, but required more water for similar yield performance. Drill sowing would allow timely establishment of the post-rice crop to utilize residual soil water in the dry season. These results suggest that mungbeans have a role as a post-rice crop with supplementary irrigation in Lao PDR and comparable environments, to improve nutrition-sensitive food security.