There had been ferry service across the Columbia at its junction with the Sanpoil since at least 1890. J. C. Keller took over the ferry run soon after a small town bearing his name was established in 1898 on the Sanpoil a dozen miles above the confluence, and the crossing became known as the Keller Ferry. The state Department of Highways began operating the Keller Ferry on September 1, 1930. Between 1939 and 1942 the water level of the Columbia gradually increased due to construction of the Grand Coulee Dam a short distance downstream. The original ferry landing was flooded in by 1939.
The Martha S was built at the nearby town of Coulee Dam, and traveled from the shipyard to the launch point on her own power. Diesel-driven propellers on both ends meant that she never had to be backed into a landing. The vessel had a 12-car capacity and was 80 feet long with a 30-foot beam. Powered by two Detroit Diesel 6-71 engines, she made the crossing in about 12 minutes. The Martha S was owned by the Washington State Department of Transportation (WSDOT; previously the Department of Highways) and operated by WSDOT's Eastern Division based in Spokane. The vessel operated seven days a week, 18 hours a day. The crew could observe both landings from one of them, and waited at a landing until a vehicle arrived, thus avoiding empty crossings.
After more than 60 years of service, and despite ongoing maintenance and upgrades, the Martha S was showing its age, and in 2012 WSDOT determined that further maintenance would no longer suffice and the ferry should be replaced. The department contracted to have a modern new ferry built, and in 2013 that vessel, the MV Sanpoil, went into service and the Martha S was retired just short of 65 years after its maiden voyage.