Clark engaged in two political campaigns that confirmed her resolve to change the country through her teaching. In 1919, she returned to Charleston and became active in the campaign to force the state to hire black teachers in the public school system. She joined the NAACP and volunteered her time, playing a key role in collecting more than 10,000 signatures. "I remember the number because of the fact that a white legislator named One-Eyed Tillman had declared [that we] would never be able to get 10,000 signatures." Soon after, a law allowing back teachers to be hired passed. "We had been victorious in this, my first effort to establish for Negro citizens what I sincerely believed was no more than their God-given rights." Soon, she herself was teaching in the Charleston public schools.
The second campaign came years later when she joined the effort to secure equal pay for black teachers, who were paid about half as much as white teachers with equivalent education and experience. She gathered affadavits and evidence, working with Thurgood Marshall, a young lawyer from the NAACP who would go on to be the first African-American Supreme Court Justice. The courts decided in their favor and, after passing a newly mandated examination, her pay tripled. (Years later, after the landmark Brown v. Board decision, Clark would be fired from the school system because she refused to give up her NAACP membership as required by a hastily passed South Carolina law.)
Clark continued to teach and be politically involved, even as her personal life evolved. In 1919, she met and married a handsome Navy cook named Nerie Clark. It was a difficult marriage: Her mother initially disapproved, their firstborn died, and she eventually found out that he had a mistress. Clark ended the marriage but never succumbed to self-pity or anger. When Nerie Clark died some 50 years later, she "pieced together all the sweet notes he had written me so the minister could help the family feel good." She lovingly cared for their son, Nerie Jr., and later her granddaughter, Yvonne. She would never marry again.
In 1953, Clark found the place that would enable her to become the engine of change that she did: Highlander Folk School. Located in rural Tennessee, Highlander was home to integrated workshops that focused on fighting racism. Clark loved the school's daring optimism and delved into the curriculum. The director, Myles Horton, recognized the value of her teaching style and asked her to come back often. Soon, she was developing pamphlets such as "A Guide to Action for Public School Segregation" and running workshops on her own. Eventually, she left her teaching job to be Highlander's education director.