In the presence of dominant shareholders, it remains uncertain whether the introduction of cumulative voting (CV) in board elections can elevate board representation of non-controlling substantial shareholders and curb the expropriation of minority shareholders by dominant shareholders. With hand-collected director-level data, we conduct DID-style analysis of China's CV reform. We find that non-controlling substantial shareholders cooperated in voting to raise their board representation, and CV implementation curbed tunneling activities and enhanced firm value. The results are especially strong in a subsample of firms whose second largest shareholder has a sufficiently large ownership proportion to elect her/his favored candidates onto boards.