In Broadhorns! Early Trade on the Mississippi players take the role of merchants based in St. Louis, financing cargo expeditions down the Mississippi River during the early part of the 19th century. The game is played over the course of a year through the varying seasons, each presenting its own challenge. During the game, the players will finance a number of trips. With each one they will need to obtain a boat (known as a broadhorn or flatboat) and purchase cargo. The player receives a higher return for their cargo the farther down the Mississippi they travel. Some of the cargo is perishable so time is of the essence. In addition, there are travelers willing to pay for passage down the Mississippi.

The players begin the game in St. Louis with a small amount of gold. They will be able to afford only the smallest of three sizes of broadhorns for their first expedition; when they have earned more gold they will choose between smaller faster broadhorns and larger broadhorns with more cargo room. They will load their broadhorns with cargo from 5 available types: apples, flour, pork, fur and whiskey. Apples, flour and pork are perishable and the players have a limited time to deliver them before they spoil. The players may stop at any town along the way and deliver their cargo and purchase more, the farther down the river they sell their cargo the greater the return. Players may also pick up travelers at various towns to earn additional gold by taking them to their destination.