Byrns worked at a corporate law firm in New York City for two years, until she left in disillusionment at how the law was practiced.[4] She drew from the experience for her 1916 essay in The New Republic, titled "The Woman Lawyer," declaring, "I do not want to practise law if it means playing a game."[5][6]
Having parted with the mainstream feminist organizations, Byrns focused her energies on pacifism, as vice-chair of the Women's Peace Society, formed in 1919, and co-founder of the Women's Peace Union in 1921.[13][14] In 1923, she and Caroline Lexow Babcock drafted a constitutional amendment to remove the power of the US Congress to declare or fund war.[15] Byrns was also on the executive committee of the War Resisters League in 1924.[16]
Byrns explained about the motivation behind her peace work, at a U. S. Senate hearing in 1927:
A government which learns to respect life will be a sane government, realizing the folly and wickedness of permitting, much less of forcing, its citizens to indulge in the abnormality of war. It will know that life, in itself valuable, can be made rich and beautiful. It will understand that its citizens can never reach the highest point of development unless they abandon such ugly practices as killing, and the violation of the personality of others, and concentrate rather on creative, constructive activities.[17]