It is located on the south side of the Columbia River and is included in the eight-county definition of Eastern Oregon.

[3] It is named for Jackson L. Morrow, a state representative who advocated for the county's formation.

The highest point in the county is Black Mountain in Umatilla National Forest at 5,923 ft (1,805 m).

No Democratic presidential nominee has won a clear majority in Morrow County since Lyndon Johnson in 1964, though Jimmy Carter and Bill Clinton both won pluralities here in 1976 and 1996, respectively (and it was just 13 votes shy of voting Democratic in 1992).

[16] However, it shifted back in 2012 and 2016, with Mitt Romney and Donald Trump both receiving roughly 65% of the vote.

The principal industries in the county today include agriculture, food processing, lumber, livestock, and recreation.

The Port of Morrow, situated on the Columbia River near the city of Boardman, was established in 1957.

Early cattlemen found an abundance of rye along the creek bottoms of the region and drove their herds into the area to forage on these natural pastures.

Increased settlement, the enclosure of the free grazing lands and diminished pastures due to overgrazing, resulted in the decline of ranching during the 19th century, and farming became predominant.

The completion of rail lines into the county in 1883 increased access to markets and encouraged wheat production in the area.

The advent of technology for center pivot irrigation has been a further stimulus to the local economy.

Two natural gas-fired plants are located at the Port of Morrow; Coyote Springs I (255-276 MW), owned and operated by Portland General Electric; and Coyote Springs II (241-280 MW; PGE operator) owned by Avista Corp.

In 2022, it was reported that Amazon intends to build at least five commercial data centers in Morrow County.