Sansom was a native of Social Circle, Georgia, and moved with her family to just outside Gadsden, Alabama, in about 1852.
In April 1863, Confederate Brig. Gen. Nathan Bedford Forrest was ordered into northern Alabama to pursue Union Colonel Abel Streight, who had orders to cut off the Confederate railroad near Chattanooga, Tennessee. On May 2, 1863, Streight arrived just outside of Gadsden and prepared to cross Black Creek. Because the creek was swollen due to rain, Streight realized that if he destroyed the bridge he could get a few hours respite from the pursuit of Forrest. Seeing the nearby Sansom farmhouse, he rode upon it and demanded some smoldering coal, which he could use to burn the bridge. When Forrest's men arrived at the site, they found the burned out bridge and came under fire from Streight's men. Forrest rode to the Sansom house and asked whether there was another bridge across the creek. Emma Sansom, then 15 years old, told him that the nearest bridge was in Gadsden, 2 miles away. Forrest then asked if there was a place where he could get across the creek. Emma told him that if one of his men would help saddle her horse, she would show him a place that she had seen cows cross the creek, and that he might be able to cross there. He replied that there was no time to saddle a horse and asked her to get on his horse behind him. As they started to leave, Emma's mother objected, but relented when Forrest assured her that he would bring the girl back safely. Emma then directed Forrest to the spot where he could cross the river. Some accounts of the skirmish indicate that the two came under fire from Union soldiers, who subsequently ceased fire when they realized that a young girl was showing them the way. After taking Emma back to her home, Forrest continued his pursuit of Streight.
Emma's heroism is noteworthy in that openly aiding Confederate forces could have subjected her and her family to prosecution (or even death) from the Union Army.
Sansom married Christopher B. Johnson on October 29, 1864, and moved to Texas in late 1876 or early 1877. She died August 9, 1900 in Upshur County, Texas, and is buried in Little Mound Cemetery.
In 1907, a monument was constructed in Gadsden at the western end of the Broad Street bridge across the Coosa River in honor of her heroism. When the residents of Alabama City, Alabama (later annexed into Gadsden) built a high school in 1929, they named it in her honor. With the consolidation of the three Gadsden city high schools at the end of the 2006 school year, General Forrest Middle School was closed and Emma Sansom High School became Emma Sansom Middle School.