There is a known association between LEGO® construction ability and mathematics achievement, yet the mechanisms which drive this association are largely unknown. This study investigated the spatial mechanisms underlying this association, and whether this differs for concrete versus digital construction. Between January 2020 and July 2021, children aged 7-9 years (N = 358, 189 female, ethnicity not recorded) completed spatial and mathematics tasks, and either a concrete or digital Lego construction task. Mediation analyses examining direct and indirect pathways (through spatial skills) between Lego construction ability and mathematics explained 8.4% to 26.6% of variance in mathematics scores. Exploratory moderated mediation analyses revealed that only the indirect path through mental rotation differed between Lego conditions. Findings are discussed in relation to theories of spatial-numerical associations and the potential of Lego training for mathematics improvement.