In solid-state physics, the ability to open and tune an electronic energy gap provides a direct route to controlling charge transport and accessing distinct electronic phases. In solids with physical boundaries, a bulk energy gap formed exclusively by extended bulk states is conventionally expected to be governed primarily by the interior bulk region, with only a minor contribution from the boundaries. Here we show that, in stark contrast to this conventional expectation, the bulk gap of armchair graphene nanoribbons in the N_w=3p+2 family with p∈Z≥ 0 can be fully controlled through the hoppings solely at the boundaries. This full boundary control originates from an exact cancellation between the gap contributions of long-range hoppings along and across the ribbon in the bulk region. It persists in the presence of Hubbard interactions and holds irrespective of ribbon width in this family. Our findings establish boundary dynamics as an unexpectedly powerful means of manipulating bulk electronic properties, enabling width-preserving engineering of one-dimensional (1D) band structures and topological phases, with potential applications in qubits, artificial quantum spin chains, topological quantum dots and other quantum devices.