Born into slavery in Fauquier County, Virginia, Newby married a woman also enslaved. Newby's father was Henry Newby, a landowner in Fauquier County. His mother was Elsey Newby, who was a slave, "owned" not by Henry, but by a neighbor, John Fox. Elsey and Henry lived together for many years and had several children, although interracial marriage was illegal in Virginia. Dangerfield was their first child. Dangerfield Newby, his mother and his siblings were later freed by his father when he moved them across the Ohio River into Bridgeport, Ohio. John Fox, who died in 1859, apparently did not attempt to retrieve Elsey, Dangerfield, or any of his siblings.  Dangerfield's wife and their seven children remained in bondage. A letter found on his body revealed the motive for joining John Brown and the raid on Harpers Ferry.
Dangerfield Newby's wife, Harriet Newby, was the slave of Jesse Jennings, of Arlington or Warrenton, Virginia. She and her children were sold to Louisiana after the raid. Newby had been unable to purchase the freedom of his wife and seven children. Their master raised the price after Newby had saved the $1,500 that had previously been agreed on. Because all of Newby's other efforts had failed he hoped to free them by force. Harriet's poignant letters, found on his body, proved instrumental in advancing the abolitionist cause. Newby was six foot two.
On the 17th of October, 1859, the citizens of Harpers Ferry set to put down the raid. Harpers Ferry manufactured guns but the citizens had little ammunition, so during the assault on the raiders they fired anything they could fit into a gun barrel. One man was shooting six inch spikes from his rifle, one of which struck Newby in the throat, killing him instantly. After the raid, the people of Harpers Ferry took his body, stabbed it repeatedly, and amputated his limbs. His body was left in an alley to be eaten by hogs. In 1899 the remains of Newby-plus remains of nine other raiders-were reburied in a common grave near the body of John Brown in North Elba New York.
Dangerfield Newby's descendants are still alive today; Tyler Newby currently lives in a suburb of Cleveland, Ohio. Josh Newby lives in a suburb of San Francisco, California and Drew Szrom lives in Massachusetts.
The following letter was found on Dangerfield Newby's body after the failed Harpers Ferry raid:
BRENTVILLE, August 16, 1859.
I want you to buy me as soon as possible for if you do not get me somebody else will. The servants are very disagreeable. They do all that they can to set my mistress against me. Dear Husband you are not the trouble I see these last two years. It has been like a troubled dream to me. It is said that the Master is in want of monney. If so I know not what time he may sell me. Then all my bright hopes of the future are blasted. For there has been one bright hope to cheer me in all my troubles, that is to be with you. For if I thought I should never see you on this earth, life would have no charm for me. Do all you can for me which I have no doubt you will. I want to see you so much. The children are all well. The baby cannot walk yet. The baby can step around any thing by holding on to it, very much like Agnes. I must bring my letter to close as I have no news to write. You must write soon and say when you think you can come
Your affectionate Wife