For the second time in this short book, non-believers respond positively to the Lord (compare Jonah 1:16).
In ancient Israel, people often fasted during prayer and repentance in difficult times (see 2 Samuel 1:12; Nehemiah 1:4). Wearing burlap and sitting on ashes often showed mourning and deep repentance (see Genesis 37:34; Job 16:15; Lamentations 2:10). The Assyrians had similar customs. These actions allowed people to show their grief in a visible way for everyone, including God, to see.
The repentance of the Ninevites criticized the stubbornness of people in the time of Jesus (Matthew 12:41).