Summary: The idea is widespread and has been published in reliable news outlets, but professional historians seem to regard it as a myth, and it may even be the case that 'lawn jockeys' were not manufactured until after the relevant historical period (and therefore could not have been used for this purpose).
A Washington Post article supports the idea that a flag tied to a 'lawn jockey' was a friendly signal to fleeing slaves. It doesn't support the idea that that's the original or only purpose or association of the statues.
In a 1984 National Geographic cover story on the underground railroad, Blockson told how the wife of U.S. District Judge Benjamin Piatt had tied a flag to a lawn jockey as a signal to fleeing slaves that it was safe to stop there.
Blockson also came across the Revolutionary War legend of Jocko. The story goes that a 9-year-old New Jersey farm boy named Jocko sneaked out of his house to find his father, a freed slave who had enlisted with George Washington's army.
The boy wound up in an encampment on Christmas Eve, before Washington's crossing of the Delaware. Waiting for his father's return, the boy volunteered to care for the general's horse during a blizzard. The next morning, Washington discovered that the boy had frozen to death, his hands still clinging to the horse's reins.
Earl Koger Sr., an African American publisher and insurance executive from Baltimore, recorded the tale in a 1976 illustrated children's book, "Jocko: A Legend of the American Revolution."
Koger's book notes that Washington was so moved by the boy's sacrifice that he ordered a likeness of Jocko placed on his lawn.
Whatever its origin, the lawn jockey became a symbol of obedient devotion -- and nowhere more welcome than among slaveholders. After the Civil War, however, the figure acquired surprising new associations, Adams said.
By the end of the 19th century, blacks dominated the "sport of kings," with black jockeys having won 15 of the Kentucky Derby's first 28 runnings, and the lawn jockey had become a totemic figure. Keeping one around might have been no more unusual than having a Michael Jordan bobblehead today.
Over time, the stooped lawn jockeys, often with cartoonish features, gave way to more erect, realistic figures -- a change that tracked advances by blacks in American society, Adams said.
But as the civil rights era began, lawn jockeys seemed like embarrassing throwbacks, and many people got rid of them. Only in recent years has interest in them increased, including among African American collectors.
Washington Post: In a Simple Lawn Ornament, Echoes of Slavery, Revolution, Fredrick Kunkle, Sunday, September 17, 2006
Looking into the matter in more detail however, research casts doubt on the idea that lawn jockeys even existed before the Civil War.
Kate Clifford Larson is a historian who spoke at the National Parks Service Network to Freedom's third National Underground Railroad Summit in 2009, on the idea that 'lawn jockeys' were related to the underground railroad:
More provocative were presentations by Kate Clifford Larson, a Simmons College professor and author of the Harriet Tubman biography, Bound for the Promised Land, who debunked the myth of the Underground Railroad’s use of the lawn jockey, ...
Larson traced the origin of the use in the Underground Railroad of the lawn jockey, the familiar ornamental lantern-holding figure. The story’s popularity stemmed from a 1963 pamphlet, “The Legend of Jocko,” written by Earl Kroger, a Baltimore insurance agent.
The lawn jockey is alleged to symbolize a 12-year-old black boy named Jocko who held the horses of George Washington’s army near Trenton, N.J., when it crossed the Delaware River on Dec. 24, 1776. According to legend, Jocko froze to death and Washington had a statue erected outside Mount Vernon in his honor. Historian Charles Blockson added credibility to the story by including it in his 1975 book, Pennsylvania’s Black History.
However, Larson found no support for its use in the Underground Railroad nor for the story attributed to Washington, which she said is simply oral tradition.
In tracing the story of its use in the Underground Railroad, she went back to the 1951 book, Ohio’s Underground Railroad Mysteries, by historian Wilbur Siebert, who told the story of a lawn jockey used by the Piatt family in West Liberty, Ohio, to signal fugitive slaves that it was a safe house. What astonished Larson was that Siebert’s early research had turned up contradictory information that included an 1852 newspaper report and mention in an 1856 book that told of an incident during which the Piatt family actually tried to obstruct the movement of fugitive slaves trying to reach Canada.
A Piatt family descendent told Larson that the story of their lawn jockey, which remains on display at a museum now operated by them and often told by a family ancestor, was probably the source of the story used by Siebert. However, Larson’s research found that lawn jockeys were not manufactured until 1865, after the period of the Underground Railroad. Her conclusion was a cautionary message that sometimes people reinvent history to separate themselves from unpleasant past associations, like slavery.
Antique Trader: Understanding the Underground Railroad: New research changes perceptions, Tom Calarco, October 7, 2009
In a letter to the Clarkston News, Ms Larson puts forward her argument:
Statue not racist, not Underground Railroad either
I read with great interest the recent article regarding the local controversy over the appropriateness of a black lawn jockey sited in front of an historic home ("Statue irks but is historical," Sept. 11).
As an historian who specializes in American slavery, the Underground Railroad (UGRR) and the pursuit of freedom, I can assure you the lawn jockey statue was never used as a signal on the UGRR.
The statue was first designed and manufactured after the Civil War, making its use as a tool to guide people to freedom impossible. The myth of the lawn jockey first appears in 1951, but takes on greater importance during the 1980s with Mr. Charles Blockson's unsubstantiated claims.
A quick call to the Blockson Collection at Temple University reveals there is no primary source documentation for this story. The Cousens are like many people who own these statues - they are not racist.
They are, however, unaware of the historical and cultural context that makes the statue a painful reminder of the Jim Crow era. Mr. Fetzer is entitled to his complaint. These vestiges of our nation's discriminatory and racist past belong in museums and not on front lawns.
Kate Clifford Larson, Ph.D.
Letters to the Editor, Clarkston News, September 18, 2013 (emphasis mine)
On the mailing list 'Underground Railroad Research Forum', Larson provides a copy of her letter to the Charles Blockson Collection at Temple University:
I have heard from the Charles Blockson Collection at Temple University. Although I had been led to believe that the Collection held positive primary documentation for the Lawn Jockey/Jocko statue as an UGRR signal, they apparently do not. ... this was my response to their prompt reply and help in clearing this issue up:
"Dear Aslaku Berhanu,
Thank you so much for your detailed response. It seems then that there is a short trail of late 20th century reference to this phenomenon of the lawn jockey myth. Earl Koger was the first person to publicize this story of "Jocko" the faithful black boy who froze to death holding George Washington's horse as Washington crossed the Delaware in 1776, and the statue that was derived from the boy's death. No where in 200 years of history had the story ever been recorded. Mt. Vernon's extensive records show no statue to a black boy, nor in Washington's extensive records, letters, journals, etc., nor of any of his staff (military or otherwise.) There is no record of a Thomas Graves in the Continental Army (and there are detailed records of black volunteers and fugitives who joined). The lawn jockey statue was never called "Jocko" until after the 1970s when Koger finally published his story. The statues, which began manufacture around late 1864 and 1865, were originally hitching posts in the form of an enslaved boy, in tattered clothes, sitting atop a bale of cotton with his hand outstretched with a ring to hitch a horse. They were sold primarily in the New Orleans market (then occupied by the Union). The statue evolved over the next few years, so that by the 1870s, it had taken the form of a black groomsman The statue was known variously as the "Sambo Hitching Post", "Negro Boy Hitching Post", and "Nigger Hitching Post". "Lawn Jockey" doesn't appear until the 20th century.
But the "Jocko" story, I believe, is a separate phenomenon from the purported use of the groomsman statue as an UGRR signal. Since your collection does not have the primary documentation that I was led to believe you have, then I must conclude that the story originated with Wilbur Siebert in his 1951 volume "Mysteries of the Ohio's Underground Railroad," in which he claims that the Piatt's of West Liberty, Ohio, ran an UGRR station. Oddly, though he identifies the Piatt family of West Liberty, Ohio, as pro-slavery in his research files used for his 1898 original Underground Railroad book, it appears that he ignores that research and claims, over fifty years later, that the Piatt's were UGRR agents, who used a lawn jockey statue as their safe house signal. His own research says this is not true, and the lawn jockey he describes and shows in his book could not have been manufactured until well after the Civil War, and possibly the 20th century. I cannot account for Siebert ignoring his own 1st person eye-witness testimony from the 1890s - which, by the way, identifies and credits numerous black UGRR operators in that region.
The Pennsylvania curriculum guide seems to merely repeat Koger's unsubtantiated work and Mr. Blockson's work, which relies on Koger's work.
I will send along a copy of my article once it is completed, for your files. I know that many people may be disappointed with this re-examination of the Jocko lawn jockey UGRR signal story, but it important not to perpetuate myths. The great tragedy is that the original hitching post statue was an incredibly racist representation of the enslavement of young children, evolving into the black groomsman who replaced real enslaved men who were now free, but whose service and embodiment of white wealth and power could now only be had in a statue - and could now be owned by thousands of middle class Americans who could no longer have a slave of their very own.
Re: Piatt of Ohio, Underground Railroad Research Forum, Kate Larson, 5/2/2008, 12:31 pm (emphasis mine)
In "The Underground Railroad in Michigan", CE Mull dismisses as 'mythological stories' the idea that a quilt or a 'lawn jockey' was a signal of the underground railroad:
While documented accounts of self-emancipation from enslavement are truly compelling, the
Underground Railroad is most often presented through fictionalized, mythological stories of escapes. Generalizations about tunnels and quilts engage our attention. Placing the focus on rare objects of popular interest obscures the verifiable history of people and events. It is easy to view the history of slavery in broad brushstrokes of recognizable patterns. But easy isn't honest. Among the hundreds of narratives and biographies cited in this book, not a single one includes any mention of a quilt or lawn jockey, referenced by recent writers as used to guide freedom seekers.
Mull, Carol E. The Underground Railroad in Michigan. McFarland, 2015, p.1.