Tillamook County, the twelfth county in Oregon to be organized, was established on December 15, 1853, when the Territorial Legislature approved an act to create the new county out of an area previously included in Clatsop, Yamhill and Polk Counties. Boundary changes were enacted with Clatsop County (1855, 1870, and 1893), with Lincoln County in 1893, Washington County (1893, 1898), and with Yamhill County in 1887.

The Coast Range behind Tillamook was the scene of a repeated series of forest fires called the Tillamook Burn between 1933 and 1951. In 1948, a state ballot approved the sale of bonds to buy the burned-over areas and have the state rehabilitate the lands. The state lands were renamed the Tillamook State Forest by governor Tom McCall on July 18, 1973. By the end of the twentieth century, the replanted growth was considered mature enough to be commercially harvested.

The Tillamook airbase for blimps was commissioned on December 1, 1942, as a U.S. Naval Air Station. The two hangers were closed after World War II and sold. One of the hangers was destroyed by a fire and only two posts now remain. The surviving blimp hangar is a local landmark and aircraft museum.

Development along U.S. Route 101 to the north of Tillamook during the last part of the 20th century has blocked part of the flood plain of the Wilson River, contributing to repeated winter flooding in the city.