<p>This study explored the effect of mapping rule switching on motor preparation in young and older adults. Motor preparation was indexed by performance in the finger cuing task, which is a cued 4-choice reaction time (RT) task requiring a single keypress with 1 of 4 fingers (index and middle fingers of both hands). Mapping rule switching required switching between two possible mapping rules implemented via spatially compatible procues and spatially incompatible anticues. These informative cues preceded the target signal at five different time intervals (100–850 ms) to assess the temporal dynamics of preparatory control relative to a non-informative (control) cue. In the single-mapping condition, procues and anticues were administered in separate trial blocks. In the mixed-mapping condition, procues and anticues were randomly intermixed, with a mapping rule cue appearing at trial onset. Analyses of (absolute) RTs and (proportional) cuing effects in single-mapping and mixed-mapping conditions revealed greater preparation benefits for procues than anticues (only at short preparation intervals), and smaller preparation benefits for older than younger adults (only at longer preparation intervals). In both age groups, switching between mapping rules in the mixed-mapping condition created mixing costs (relative to single-mapping), reflecting substantial deficits in motor preparation, and more so at longer preparation intervals where proactive control dominates. These findings reveal a strong impact of mapping rule switching on motor preparation. We propose that activating a new mapping rule and preparing an action both require updating operations in working memory that bias response selection mechanisms.</p>