According to the 2010 census, its population was 34,387, an increase of 2.27% from 33,625 in 2000. The county has four incorporated cities and towns with a total population of over 15,000, as well as many small unincorporated communities. The county is divided into 12 townships which provide local services. There are four Indiana state roads in the county, as well as three U.S. Routes and one railroad line.
Adams County was created by statute on February 7, 1835, and organized March 1, 1836. Its name honors the sixth President of the United States, John Quincy Adams. The site of the county seat was chosen May 18, 1836.
The first courthouse was a two-story frame building and was commissioned in 1839. The first jail was a log building that was completed in 1837. The present Adams County courthouse was built in Decatur in 1872–1873 at a cost of $78,979. The designer was J. C. Johnson, who had been trained as a carpenter and joiner and became a self-taught architect; he won second place in the Indiana State Capitol design competition. The construction was done by Christian Boseker of Fort Wayne. It is built of red brick with stone ornamentation.