What Really Started the Great Chicago Fire of 1871?

The Great Chicago Fire raged for two days in October 1871, killing 300 Chicagoans, leveling 17,000 buildings and leaving more than 100,000 people homeless. The Great Chicago Fire wasn’t the deadliest in U.S. history—the Peshtigo Fire, which burned at the same time, killed more than 1,200 people in Wisconsin—but it’s easily the most famous.  

Equally famous is the story of who started the Great Chicago Fire: Mrs. O’Leary—and her cow, of course. 

On October 9, 1871, as the Great Chicago Fire was still burning, the Chicago Evening Journal reported, without proof, that the blaze began with “a cow kicking over a lamp in a stable in which a woman was milking.” 

Multiple eyewitnesses later testified that they saw the first flames coming from the barn owned by Patrick and Catherine O’Leary, Irish immigrants. The O’Learys swore under oath that they were asleep when the fire began. And Catherine, who was cruelly caricatured by the press as a callous hag, told officials that she last visited the barn hours earlier and didn’t carry a lamp.  

Even though no solid evidence links Mrs. O’Leary or her cow to the Great Chicago Fire, the two have taken the blame for more than 150 years, at least in the popular imagination. But there are other theories as to who and what really caused the Great Chicago Fire, including a city that was seemingly built to burn.  

The summer and fall of 1871 were two of the driest and hottest seasons in Chicago’s history. The entire Midwest was plagued by a months-long drought that created tinder-dry conditions. The Great Chicago Fire was only one of dozens of blazes to strike Chicago during that historic dry spell. In the first week of October alone, Chicago firefighters put out nearly 30 fires, including the so-called Saturday Night Fire that burned on October 7, the night before the Great Chicago Fire.  

The parched weather conditions, however, were only partly to blame for the rash of fires in Chicago, says Carl Smith, a Chicago historian and author of Chicago’s Great Fire: The Destruction and Resurrection of an Iconic American City. The biggest reason was that the city was built like a tinderbox. 

“Chicago in 1871 was almost entirely made of wood,” Smith says. “Not only were buildings made of wood, but there were wooden streets, raised wooden sidewalks and wooden fences everywhere, including in the O’Learys’ neighborhood.”  

In 1871, more than 300,000 people lived in Chicago, and there were minimal building codes, no fire safety regulations and no city officials to enforce them. In his book, Smith writes, “It was as if Chicagoans had purposely intended to construct a city they wished to burn down and then voted for aldermen and mayors who did little to stop that from happening.” 

The Saturday Night Fire that preceded the Great Chicago Fire started in the boiler room of an urban lumber mill and quickly grew into a raging inferno fed by piles of boards and sawdust. All of Chicago’s roughly 190 firemen spent the long night of October 7 battling the Saturday Night Fire, which consumed four city blocks, injured firefighters and damaged their equipment. 

Smith says that almost any of the more than two dozen fires the previous week could have easily blown up to become the Great Chicago Fire. What happened on the night of October 8 was simply a “perfect storm” of adverse circumstances: “A long dry summer, an already understaffed fire department further weakened and depleted from fighting a major fire the night before, a strong wind blowing toward the heart of downtown on an unseasonably hot October night and a breakdown in what was a state-of-the-art alarm system. All of these factors came together.”  

All that was needed was a spark.  

The O’Learys lived on DeKoven Street in a densely populated West Side neighborhood of one-story shacks, “every wretched building within four feet of its neighbor, and everything of wood,” wrote a local reporter.  

Catherine and Patrick O’Leary and their five children shared a small cottage, part of which they rented to another Irish family. Patrick earned meager wages as a day laborer, and Catherine supplemented their income by selling milk from their five cows, which they kept in a small barn out back.  

Around 9 p.m. on October 8, 1871, the O’Learys were in bed when they were awakened by a neighbor, Daniel “Peg Leg” Sullivan, shouting “Fire! Fire!” In his testimony at an official inquiry, Sullivan said that he saw flames shooting from the O’Leary’s barn and attempted to rescue the animals, but he slipped on his wooden leg and barely escaped with his life.  

The claim, first published in the Chicago Evening Journal, that the fire started when Mrs. O’Leary’s cow kicked over a kerosene lamp, was completely fabricated, but it spread as quickly as the fire itself.  

On October 13, while Chicago newspapers were still recovering from fire damage, The New York World elaborated on the story, claiming that Mrs. O’Leary had received welfare checks until the city learned that she owned five cows. According to the World, Mrs. O’Leary visited her cows every evening with a lamp in hand. That fateful night, she went back to the house for some salt, and when she returned, the barn was in flames.  

When the Chicago Times resumed publication on October 18, the paper doubled down on the characterization of Mrs. O’Leary as a welfare cheat who “swore she would bring revenge on a city that would deny her a bit a wood or a pound of bacon.” Although Catherine was only 40 years old in 1871, the Times described her as “about 70 years of age and... bent almost double with the weight from many years of toil, trouble and privation.”  

Catherine, defending herself in an interview with the Chicago Journal, denied taking a cent of welfare money and said there wasn’t a word of truth to the lamp story. “I always milked my cows by daylight and never had a lamp of any kind or a candle about the barn.”  

Even after an official inquiry cleared Catherine O’Leary of responsibility, the “cow kicking over a lamp” myth proved irresistible. It provided a simple explanation for a complex event that was more about poor city planning than an individual culprit.

“People wanted a clear reason why [the Great Chicago Fire] happened, and one that reflected what they already believed,” Smith says. “So it becomes about blaming this poor Irish immigrant woman. Since then, it transformed into a more benign piece of urban folklore—the woman who went out to milk a cow and burned down a city.” 

In a report issued on December 11, 1871, Chicago’s Board of Police and Fire Commissioners determined that the exact cause of the Great Chicago Fire was unknown, concluding that it could have been “a spark blown from a chimney on that windy night, or was set on fire by human agency.”  

Meanwhile, the myth of Mrs. O’Leary and her cow persisted. The fictitious tale took on new significance as Chicago rebuilt in the decade following the fire. In the retelling of Chicago’s rebirth and revitalization, the “foolish” characters of Mrs. O’Leary and her cow (posthumously named Daisy) became the literal spark that remade the city.

That didn’t stop people from searching for the true cause of the Great Chicago Fire. In 1985, educational filmmaker Melvin Waskin published Mrs. O’Leary’s Comet, a book claiming that both the Great Chicago Fire and the Peshtigo Fire were sparked by the same celestial source.  

According to Waskin’s theory, a fragment of Biela’s Comet struck somewhere in the greater Chicago region on October 8, 1871. Waskin claimed that major fires spread so quickly across the Midwest because they were ignited by large hunks of the comet that broke off and caught fire in the atmosphere. While Waskin’s theory was creative, it didn’t explain why there were no reports in 1871 of a light streaking through the sky or any evidence of an impact crater.  

Another theory is that Daniel “Peg Leg” Sullivan, the neighbor who first spotted the fire in the O’Leary’s barn, was actually the guy who started it. The case against Sullivan was built by amateur historian Richard Bales in his book, The Great Chicago Fire and the Myth of Mrs. O’Leary’s Cow. By comparing Sullivan’s testimony with the physical layout of DeKoven Street, Bales concluded that Sullivan lied about his whereabouts on the night of October 8. Was Sullivan trying to cover up the fact that he accidentally ignited hay with sparks his own pipe while stealing milk? That’s Bales’s theory.  

“I don’t think it’s true,” says Smith, “But if it is, the question remains: what kind of a city can be burned down by a man smoking his pipe?” 

As for Mrs. O’Leary, she finally received an official reprieve in 1997. The mayor and City Council of Chicago voted unanimously to “forever exonerate Mrs. O’Leary and her cow from all blame,” recognizing that she was unfairly “vilified and maligned by history.”