This mixed-methods, quasi-experimental one-group pre–post study investigated whether a structured sequence of unplugged computational thinking (CT) activities embedded in regular primary mathematics lessons was associated with measurable changes in pupils’ CT practices and how such changes manifested in classroom interaction. Participants were 150 grade 4–5 pupils in a Vietnamese public primary school who completed eight 45-min, screen-free lessons aligned to five CT practices: decomposition, pattern recognition, abstraction, algorithm design, and evaluation/debugging. Quantitative analyses revealed statistically significant pre–post improvements across all five CT subscales (all p < .001 ), with large effect sizes (Cohen's d = 2.58 − 3.29 ). Gains were consistent across practices, indicating broad within-cohort growth rather than isolated skill shifts. Qualitative evidence from structured observations, student interviews, and artifact analysis converged with these results, documenting increased representational coordination, more explicit articulation of stepwise procedures, systematic test–revise (debugging) cycles, and distributed collaborative monitoring. Joint analysis linked subscale gains to three mechanism clusters: (i) representational fluency that stabilized invariants for inspection and generalization, (ii) iterative strategic refinement through visible debugging routines, and (iii) collaborative regulation enacted through role rotation and peer verification. Within the bounds of a classroom-based design, the findings provide convergent evidence that low-cost, unplugged tasks can be integrated into routine mathematics instruction and are associated with substantial growth in taxonomy-referenced CT practices in a resource-constrained setting.