Charles Duncan Gilfillan practiced law in St. Paul with his older brother James Gilfillan, the future Chief Justice, from 1857 to 1863, when he went into business. For the next thirty years, he was a successful banker, real estate developer and entrepreneur in St. Paul.
He was also one of the founders of the Republican Party in Minnesota and was chosen the first chairman of its central committee in 1856. He was a fervent party activist from that time to his death in 1902 at age seventy-one. He was a keen observer of politicians and, as the years passed, developed warm feelings toward most of them, foes as well as friends.
In 1898, he recalled several politicians and several political battles of the territorial era, including the constitutional convention in 1857, in an address to the Executive Committee of the Minnesota Historical Society.