In Asia, some children are taught a calculation technique known as the ‘mental abacus’. Previous research indicated that mental abacus experts can perform extraordinary feats of mental arithmetic, but it disagrees as to whether the technique improves working memory. The present study extended and clarified these findings by contrasting performance from several numerical and working memory tasks across three groups of participants: Japanese mental abacus experts, abacus-naïve Australian undergraduates, and abacus-naïve Japanese undergraduates. It also investigated whether the mental representations and strategies used to process two-digit numbers differed across the three groups. First, the results showed that the Japanese mental abacus experts only performed better when the numerical and working memory tasks involved arithmetic problems, suggesting domain-specific transfer rather than domain-general improvements to numerical processing or working memory. Second, the results suggest that the Japanese mental abacus experts were less reliant on decomposed magnitude representations, and used a processing strategy that is less sensitive to the perceptual overlap between numbers. Finally, performance was less discrepant between the Australian and Japanese abacus-naïve undergraduates than either group with the Japanese mental abacus experts, indicating that mental abacus training, rather than socio-cultural differences, was responsible for the observed group differences.