[2] The county was created on May 2, 1885,[3] and was named for Marshall Vincent, who homesteaded near Andover, South Dakota.

Its terrain consists of rolling hills, with numerous lakes and ponds in the SE portion.

[5] Its terrain slopes to the northeast, and its highest point is near its SE corner, at 2,034 ft (620 m) ASL.

[8] A portion of the Lake Traverse Indian Reservation is located in the eastern part of the county.

Marshall County voters have tended to vote Democratic for the past several decades.