Janney became a prominent member of the Virginia chapter of the American Colonization Society, and was its president several times in the 1850s. In 1847, Janney was one of three lawyers who defended Nelson Talbott Gant, a freed slave from Leesburg, who was accused of stealing his wife, still a slave, from her owner after the owner had refused to allow Gant to buy her freedom. Janney and his colleagues obtained Gant's acquittal by arguing that the bonds of marriage transcended those of slavery.