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- Mahaska County was formed on February 17, 1843. The county has been self-governing since February 5, 1844.
- It was named after Chief Mahaska of the Iowa tribe.
- The county was the first in Iowa to have a sheriff and a justice of peace.
- The first courthouse was completed in January, 1846. The second structure was put into use on February 27, 1886.
- The first school, a small log cabin one-half mile east of Oskaloosa, was opened on September 16, 1844 and the Cumberland Presbyterian Church opened as the first church in 1846.
- On July 2, 1850, the first edition of the Iowa Herald was issued (today the Oskaloosa Herald).
- In 1864, the first tracks were laid through the county of the Des Moines Valley Railroad.
- Coal mining was once a major industry in Mahaska County. In the 19th century, Muchakinock, about 5 miles south of Oskaloosa, was probably the largest and most prosperous coal camp in Iowa.
- County Seat: Oskaloosa
- Mahaska County’s population: 22,181 (from the July 1, 2016 estimate of the Census Bureau.)