The constitutional amendment mechanism of the 1857 and 1917 Mexican constitutions mimic Article V of the U.S. Constitution in important ways. The Mexican experiment, however, functioned very differently than its American counterpart. While the amending procedure in the U.S. has resulted in incremental, slow, informal constitutional change through a consequential and robust judiciary, the dynamics of constitutional reform in Mexico has resulted in frequent and fast formal constitutional change and a sidelined and weak pushing. This paper offers an account explaining the divergent paths of constitutional change in these two countries. The author uses a historical account to argue that amendment difficulty is not only explained by the way institutional rules are designed but mostly by the party system, political structure, and constitutional culture.