The fire began at 9:00 PM on October 8th in a small barn belonging to the O’Leary family. The cause of the fire is still unknown to this day; the popular story is that an O’Leary cow knocked over a lantern. Chicago had been suffering from a drought, and over 70% of the buildings, as well as the sidewalks, were made of wood, with the roofs of the buildings being highly flammable. The fire spread rapidly, and by the time Chicago’s understaffed fire department could respond, it had enveloped many nearby buildings. Initially, it was hoped that part of the Chicago River would act as a barrier to further spread, but the fire was so hot and the wind so strong that it quickly jumped the river and spread to the center of the city. The fire was so intense that it created its own tornado-like winds, which helped spread the fire with frightening speed. Firefighters continued to battle the blaze until the waterworks caught fire and burned down. When the waterworks burned, the firefighters lost their ability to fight the fire and could only watch as it burned until it reached the edges of the city and it began to rain. By that time, there was little left to burn.

Once the fire petered out and it was safe to survey the damage, the devastation became clear. An area 4 miles long and 3/4 mile wide had been destroyed. Seventy-three miles of road and 17,500 buildings were gone. One hundred thousand people, about one-third of the city's total population, were homeless. After the fire, 120 bodies were found, although the death toll might have been as high as 300. The total value of the property loss was $222 million, or over $4 billion in today’s dollars. Help for Chicago streamed in from all over the world, and the city rebuilt.