On March 11, 1848, a bill was passed by the General Assembly of Virginia to form Putnam County from portions of Kanawha, Mason, and Cabell. The new county, which straddles the Kanawha River midway between Charleston and Point Pleasant, was named in honor of Gen. Israel Putnam, who commanded the Continental Army at Bunker Hill. The 1850 census showed a population of 5,336, including 632 slaves. Putnam, with 350.3 square miles of river valleys and rolling hills, was entirely a farming county until the late 1800s.

The town of Winfield, named for Gen. Winfield Scott, was established as the county seat when the first county meeting was held. Winfield, a river town with no railroad or paved road leading to it until 1930, was slow to expand. A devastating fire in 1928 destroyed one of the main town blocks. After the Winfield navigation dam was completed in 1936 and the new Kanawha River bridge was opened in 1957, the town became a progressive, growing community.

When the Civil War cast its shadows on the Kanawha Valley, it was apparent that Putnam would be a divided county within a divided state. Friends, neighbors, and even family members separated over the issues of the day. Enlistments were equally divided, as about 400 Putnam citizens joined each side. There were four significant engagements in Putnam County, at Scary Creek, Red House, Hurricane Bridge, and Winfield, and Putnam men served on many battlefields far from home.

Coal was discovered on the Pocatalico River in 1798, but it was not mined commercially until the late 19th century. In the early 1900s, five mining companies were operating between Raymond City and Plymouth. By 1907, Putnam County mines employed 1,000 men and produced more than 400,000 tons of coal a year. Much has been written about the grim living conditions and labor struggles in the West Virginia coalfields, but Putnam seems to have been spared the worst of it. At the beginning of World War II, the county mines shut down. The younger miners went into the service, and the older men with families found war-time jobs at the Nitro plants and other locations.

Older residents will remember that the Kanawha River divided the county during the age of railroad and highway transportation and before the river bridges were built. The only way to cross the Kanawha was by the unpredictable Winfield ferry. But once, the river had united Putnam County. While the steamboat trade flourished, the division of the county was not much of an issue. People rode the daily packet boats to and from Charleston, and some were even picked up at their own landings. This created a popular social culture as passengers visited with one another in the dining rooms and along the deck rails. Lasting friendships were established from both sides of the river.

With the coming of the railroads, river travel diminished. With coal mining on the north side of the river and farming on the south side, the county became divided politically, economically, and socially, as well as geographically. Public improvements usually benefited only one side, which kept most bond issues from passing and caused the county to fall behind.

Finally, with the completion of the Winfield Bridge and Interstate 64, Putnam County began to prosper. The county created a planning commission, and eventually roads were improved, utilities extended, and areas zoned for housing and commercial and industrial development.

Now Putnam has thriving communities on both sides of the Kanawha River. They include Red House, which was named for a house-size red rock that can be seen high on the hill behind the town. Nearby Eleanor was built in 1934 as a federal project to relocate displaced families to small homestead farms. By the end of World War II, the Eleanor families had bought out the federal government's interest and established their own municipal government.

Buffalo, on the north side of the Kanawha River near the Mason County line, was laid out and incorporated in 1837, making it the oldest established town in Putnam County. Buffalo was originally a farming community, but with the location of the Toyota plant there in the 1990s, it has become a manufacturing and rural residential center.

Bancroft, settled in 1848 and first named Energetic, was located three miles below the mouth of Pocatalico River, or Poca River as it is commonly known. Many Plymouth mine employees lived at Bancroft. Nearby Poca was a trade and shipping center for Poca River farmers until after the Civil War. Many Raymond City miners preferred to live in Poca. In 1895, a timber company began extensive operations on the waters of Pocatalico, which added to Poca's growth.

On the south side of Kanawha River, Hurricane lies in the middle of Teays Valley, halfway between Huntington and Charleston. It was established in 1874 as Hurricane Station after the Chesapeake & Ohio (C&O) Railway was completed. Surrounded today by growing suburbs, Hurricane is the center of one of the fastest-growing areas in West Virginia. Farther east in Teays Valley, Scott Depot includes the area that extends from Scary Creek Hill to State Route 34. Scott Depot was an early stop on the C&O, and construction workers were housed there during the building of the railroad.

Nitro, a World War I boom town, was completed in 11 months in 1918. The town on the Putnam-Kanawha county line had 25,000 people, and the plant was producing 700,000 pounds of gunpowder per day when the war ended in the same year. The entire Nitro site was bought by a development company and its industrial facilities sold off, the beginning of a prosperous chemical industry in the Kanawha Valley.

By 1998, the 150th anniversary of Putnam County, all previous expectations for growth and development had been exceeded. The Toyota plant had been completed at Buffalo, a warehouse complex at Nitro was in operation, and the Rock Branch Industrial Park was at full capacity. There is a new Kanawha River Bridge at Buffalo, and a section of Route 35 has been upgraded to four lanes and connected to I-64 just east of the Winfield interchange. Putnam is one of the fastest growing counties in West Virginia, increasing by more than 11 percent during the 21st century. The 2020 population was 57,440, more than double its total from 50 years before.

In 2023, Putnam County's largest employers were Toyota, the county school system, Vandalia Health (CAMC), Appalachian Power, and Walmart.