Many skills that humans acquire throughout their lives, such as playing video games or sports, require substantial motor
learning and multi-step planning. While both processes are typically studied separately, they are likely to interact during
the acquisition of complex motor skills. In this work, we studied this interaction by assessing human performance in a
sequential decision-making task that requires the learning of a non-trivial motor mapping. Participants were tasked to move
a cursor from start to target locations in a grid world, using a standard keyboard. Notably, the specific keys were arbitrarily
mapped to a movement rule resembling the Knight chess piece. In Experiment 1, we showed the learning of this mapping
in the absence of planning, led to significant improvements in the task when presented with sequential decisions at a later
stage. Computational modeling analysis revealed that such improvements resulted from an increased learning rate about the
state transitions of the motor mapping, which also resulted in more flexible planning from trial to trial (less perseveration
or habitual responses). In Experiment 2, we showed that incorporating mapping learning into the planning process, allows
us to capture (1) differential task improvements for distinct planning horizons and (2) overall lower performance for longer
horizons. Additionally, model analysis suggested that participants may limit their search to three steps ahead. We hypothesize
that this limitation in planning horizon arises from capacity constraints in working memory, and may be the reason complex
skills are often broken down into individual subroutines or components during learning.