Johnson returned to Philadelphia from New York in August 1855 and testified in the trial of William Still and five dockworkers, charged by her master John Hill Wheeler with assault. They had aided her escape. She testified at length about having planned to gain freedom in the North, and said she chose of her own free will to leave with Still, and would never go back to slavery. She helped win acquittal for Still and three men, and reduced sentences for two others. State and local officials protected her after testimony, and she and her sons soon moved to Boston, where they settled. She married again there. Her son Isaiah Johnson served in the United States Colored Troops during the American Civil War.