He was known for his participation in labor law, civil rights, peace and justice issues, and freedom of speech cases; in particular, defending those accused of communist leanings.

[2][4] He attended Harvard Law School on a full scholarship,[1][2] receiving a Juris Doctor degree (cum laude) in 1932.

[2] Cammer interrupted his legal career to serve in the United States military during World War II.

[9] Chambers described the day as follows: On August 10th, a trio of witnesses collectively more interesting than [Henry] Collins appeared before the Committee.

[13] In 1951, Cammer joined more than half a dozen other lawyers in defending 17 Communist Party members, including Elizabeth Gurley Flynn.

Cammer was held in contempt of court in June 1954 for sending a questionnaire to potential grand jurors in the case.

[21][22][23] In 1955, when Witt left the firm to become full-time counsel to the International Union of Mine, Mill and Smelter Workers,[24][25] Ralph Shapiro (November 10, 1916 – January 9, 2014), a graduate of City College of New York and University of Michigan Law, as well as member of the American Labor Party and National Lawyers Guild, was elevated to partner, so Cammer's firm changed its name to Cammer & Shapiro.

[29][30] The National Lawyers Guild was branded a communist front by the Federal Bureau of Investigation, the United States Department of Justice, and (later) the House Un-American Activities Committee (HUAC).

[3][1] Cammer married the former Florence Glantz on January 25, 1936; the couple had two children, Robert and Margaret,[4] who was New York State Acting Supreme Court Judge and former Deputy Administrative Judge of the New York City Civil Court, as well as spouse of American painter Joan Snyder.

[32] Harold I. Cammer died age 86 on October 21, 1995, at his home in Mamaroneck, New York; he was survived by his wife, son, daughter, grandson, and two great-granddaughters.