In addition to her work in support of women's suffrage, Laidlaw was a crusader against white slavery and the forced prostitution of both white and Chinese women in New York, and was a proponent of the Mann Act of 1910. In 1912, following a violent attack on anti-prostitution activist Rose Livingston, Laidlaw and her husband helped mobilize public opinion against the perceived inaction of Mayor William Jay Gaynor in ordering increased police protection for activists in New York's Chinatown. On November 9 of that year, she served as chairman of a torchlight parade down Fifth Avenue that drew an estimated 400,000-500,000 observers, an event that solidified her position as a leader of the suffragist movement. She wrote many articles and columns, spoke at public gatherings, and traveled around the country, including a trip through the western United States in 1913 to help organize activists. Laidlaw spoke out against the notion of separate spheres for men and women in regards to public life, writing in 1912 that, "insofar as women were like men they ought to have the same rights; insofar as they were different they must represent themselves." In 1914, her most significant writing was published, a booklet entitled "Organizing to Win by the Political District Plan", which gave activists step-by-step instructions on how to fundraise and engage with their local political leaders to keep up sustained pressure in support of suffrage. Laidlaw became a director of the National American Woman Suffrage Association (NAWSA) in 1917, and was among a group of leading suffragettes who met with former President Theodore Roosevelt to persuade him to lend support to their cause. Later that year, an amendment to the New York Constitution granting women the vote was passed. Following the ratification of the Nineteenth Amendment, Laidlaw's attention turned to international relations, and she promoted the United States' entry into the League of Nations as well as the formation of the United Nations. She was a strong supporter of Prohibition and was a member of the New York State Prohibition Society. Both Harriet and James Laidlaw were honored by the League of Women Voters on a plaque unveiled in 1931 and now displayed in the New York State Capitol in Albany listing those of “distinguished achievement” in the women's suffrage movement; James is the only man listed.