The legislative power of the Borough is vested in the Assembly. The Matanuska-Susitna Borough Assembly is comprised of seven members elected from districts for staggered three-year  terms for no more than two consecutive full terms. The composition and apportionment of the assembly is consistent with the equal representation standards of the Constitution of the United States.

The Assembly approves the budget, sets the mill rate  for taxation, and appropriates funds to provide for Borough services. Through laws passed, the Assembly establishes policy statements which are then executed through the administration.