Brother to Abp. John Carroll, Daniel represented Maryland in the Colonial Congress (1780–1784), at the Constitutional Convention of 1787, and in the new Congress (1789–1791). He was one of several delegates at the Convention who supported direct popular election of senators, a method that was later implemented with passage of the seventeenth amendment in 1913. With Thomas Fitzsimons, he was one of two Catholic signers of the Constitution.