Children with dyslexia frequently also struggle with math. However, studies of reading disability (RD) rarely assess math skill, and the neurocognitive mechanisms underlying co‐occurring reading and math disability (RD+MD) are not clear. The current study aimed to identify behavioral and neurocognitive factors associated with co‐occurring MD among 86 children with RD. Within this sample, 43% had co‐occurring RD+MD and 22% demonstrated a possible vulnerability in math, while 35% had no math difficulties (RD‐Only). We investigated whether RD‐Only and RD+MD students differed behaviorally in their phonological awareness, reading skills, or executive functions, as well as in the brain mechanisms underlying word reading and visuospatial working memory using functional magnetic resonance imaging (fMRI). The RD+MD group did not differ from RD‐Only on behavioral or brain measures of phonological awareness related to speech or print. However, the RD+MD group demonstrated significantly worse working memory and processing speed performance than the RD‐Only group. The RD+MD group also exhibited reduced brain activations for visuospatial working memory relative to RD‐Only. Exploratory brain‐behavior correlations along a broad spectrum of math ability revealed that stronger math skills were associated with greater activation in bilateral visual cortex. These converging neuro‐behavioral findings suggest that poor executive functions in general, including differences in visuospatial working memory, are specifically associated with co‐occurring MD in the context of RD.Research Highlights Children with reading disabilities (RD) frequently have a co‐occurring math disability (MD), but the mechanisms behind this high comorbidity are not well understood. We examined differences in phonological awareness, reading skills, and executive function between children with RD only versus co‐occurring RD+MD using behavioral and fMRI measures. Children with RD only versus RD+MD did not differ in their phonological processing, either behaviorally or in the brain. RD+MD was associated with additional behavioral difficulties in working memory, and reduced visual cortex activation during a visuospatial working memory task.