An international lawyer, he was president of the American Peace Society and played a role in writing the Charter of the United Nations.

[6] Peaslee enlisted in the United States Army after graduation from law school, ultimately attaining the rank of Major.

In March 1918, Army General John J. Pershing authorized Peaslee to organize the Silver Greyhounds, the first U.S. diplomatic courier service, to carry sensitive correspondence between Paris and Washington, D.C. during World War One and the U.S.-led peace efforts that followed.

[7] Peaslee retained an original copy of the Treaty of Versailles which was later donated to the U.S. Department of State archives.

During World War II, Peaslee was a Commander in the United States Navy, training coast guardsmen in sabotage prevention techniques.