[2] A bill creating Hidalgo from the southern part of Grant County was passed on February 25, 1919, taking effect at the beginning of 1920.

[3][4] The county was named for the town north of Mexico City where the Treaty of Guadalupe Hidalgo was signed,[5] which in turn was named for Miguel Hidalgo y Costilla, the priest who is known as the "Father of Mexican Independence."

As of the 2000 census,[12] there were 5,932 people in the county, organized into 2,152 households, and 1,542 families.

Out of the total population, 38.90% of those under the age of 18 and 17.00% of those 65 and older were living below the poverty line.

As of the 2010 census, there were 4,894 people, 1,936 households, and 1,286 families residing in the county.

[16] Hidalgo has historically been a swing county, going for the winner every time since 1928, except for 1968 and 2020, where it went for Hubert Humphrey and Donald Trump respectively.

In 2024, Trump broke 60% of the county's vote, the first time that a Republican presidential candidate had done this since Nixon's 1972 landslide.