In 1973, he was elected to the Common Cause National Governing Board, a nonpartisan, nonprofit advocacy group and think tank with the mission to make political institutions more open and accountable. From 1974 to 1977, he served as chairman of New Jersey Common Cause. As chairman he successfully lobbied for New Jersey's Sunshine Law, which made government meetings open to the public. He also championed campaign finance reform, working closely with Thomas H. Kean, then a member of the New Jersey General Assembly. Zimmer then served as treasurer for Kean's reelection campaign.