At the turn of the twentieth century subsistence farming served as the principal occupation. Much changed in the century's first two decades. Better transportation, refrigerated railroad cars, new farm equipment, and agricultural education brought improvements. The value of dairy products rose from $127,389 in 1919 to $186,499 ten years later. Egg production increased from 390,006 dozen in 1919 to 636,835 in 1929. Over time, labor-intensive row crops and grains gave way to cattle. In 1920 the county had 2,176 farms with 257,671 acres. This held steady until the 1960s, when the number began decreasing. In the 1970s poultry, mainly broilers, became a key product. In 1997 there were 1,303 farms covering 364,620 acres. The value of agricultural products sold that year was $94 million, approximately two-thirds of which were poultry products.

Three lakes changed the face and the economy of Delaware County. In 1924 the city of Tulsa, completed a water-supply dam at Spavinaw on Spavinaw Creek in Mayes County. Lake Spavinaw impounds water into Delaware County. Although small, the lake began to attract weekenders and day-trippers. In 1940 a dam on Grand River was completed, and Grand Lake O' the Cherokees was filled. Grand Lake sprawls north across Delaware County into Ottawa County. Creation of the lake also displaced hundreds of people, covered roads, and necessitated relocation of a number of cemeteries. Sailboat Bridge on Grand Lake is the second-longest bridge over water in Oklahoma.

Tulsa's growing need for water caused it to revisit Spavinaw Creek, and in 1952 an upstream dam was completed, creating Lake Eucha. It became a popular fishing and recreation area. The lake inundated the Cherokee town of Eucha, the home of Cherokee Chief Charles Thompson, or Oochalata, who was buried there. The town and the cemetery were relocated. The chief is buried just inside the gate of the new cemetery, the grave marked by a military stone for his Civil War service to the Union.

After World War II a boom began. Tourism and retirement became big business. In 1965 Green Country, Incorporated, formed to coordinate promotion and development of sixteen northeast Oklahoma counties. In 2000 the three largest occupation groups, each at 23.4 percent of the labor force, were management, professional, and related occupations; production, transportation and material moving occupations; and sales and office jobs. Service occupations involved 15.9 percent, while farming and related jobs involved only 1.8 percent.

Rail access assisted in community growth over the years. In 1896 Congress authorized the Arkansas Northwestern Railway to build a line from Southwest City, Missouri, northwest to the Kansas line between Baxter Springs and Chetopa. Under the name of the Arkansas and Oklahoma Railroad, a line was built from Rogers, Arkansas, to Grove, reaching there in 1900. The next year, the St. Louis and San Francisco Railway bought the line, which never extended beyond Grove, and was abandoned in 1940. In 1912-13 the Kansas, Oklahoma, and Gulf Railroad built tracks through the region to connect Kansas and Texas. Upon entering Delaware County, the line looped west and then southwest, following the west bank of Grand River and exiting into Craig County. In 1912 the town of Bernice was platted on both sides of the railroad near the crossing of Horse Creek. Many businesses in Needmore, two miles east, moved to Bernice. Further east, Rabbit, a small community north of Grove, later renamed Copeland, and Switch developed. When Grand Lake was built, Bernice relocated to higher ground on the west side of Horse Creek, and some of the railway grade was moved. The line was abandoned in 1966, and the railway grade, including the Horse Creek Bridge, became State Highway 85A.

In 1910 Delaware County had a population of 11,469; only five counties had fewer people. This did not change appreciably until 1970 when the population surged. The 2000 census recorded 37,077, more than fifty-nine other counties. In 2004 the census revealed that while rural counties were losing population, Delaware County was the fifth-fastest-growing in the state. In addition to tourism, a tight labor market, aggressive action to attract employers, and a growing economy in adjacent northwest Arkansas contributed to these changes.

Since the surge of whites into Indian Territory in the nineteenth century's last decades, they have outnumbered other groups. In 1900 the census of Indian Territory recorded whites at 77.2 percent and American Indians at 13.4. The 2000 census tallied whites 70.2 percent and American Indians 22.3.

The county offers several recreation areas and historic sites. Just south of Lake Eucha is the 14,316 acre Spavinaw Wildlife Management Area. State parks include Honey Creek at Grove and Bernice at Bernice, Lake Eucha south of Jay, and Natural Falls near West Siloam Springs. Har-Ber Village, a large museum, is west of Grove. Near present Oaks is Moravian Springplace Mission. Southwest of Maysville, Arkansas, is the site of Fort Wayne. Six properties have been listed in the National Register of Historic Places. Polson Cemetery (NR 77001092), west of Southwest City, Missouri, is the burial place of Major Ridge, John Ridge, and Stand Watie. Northwest of Grove in the Seneca-Cayuga area is the still-active Splitlog Church (NR 72001061) built by Mathias Spitlog. The Hildebrand-Beck Mill (NR 72001062) stands on Flint Creek just north of U.S. Highway 412 east of Kansas, Oklahoma. The Talbot Library and Museum in Colcord displays artifacts and provides genealogy resources.

Notable natives of Delaware County include Lee B. Smith (1844-1917), an adopted Cherokee who served on the Cherokee Nation Council and was the county's first state legislator. John H. Gibson (1861-1940) owned the Grove Sun and served in local and state offices.