Grenville Clark (November 5, 1882 – January 13, 1967) was a 20th-century American Wall Street lawyer, co-founder of Root Clark & Bird (later Dewey Ballantine, then Dewey & LeBoeuf), member of the Harvard Corporation, co-author of the book World Peace Through World Law, and nominee for the Nobel Peace Prize.

The National Historic Register (US DOI, National Park Services) has called Clark an "international lawyer and legal architect of world organizations," who was "active in world peace efforts and an advisor in governmental affairs." Further, he was a "drafter of the United Nations Charter, author of A Plan for Peace and co-author of the acclaimed World Peace Through World Law. He was an advisor to four United States Presidents, founder of the Military Training Camp Association (1917) and leader of the Plattsburg movement, and author of the Selective Service Act of 1940. Clark organized the two Dublin Peace Conferences, held at the Morse Farm in 1945 and 1965, out of which grew the United World Federalists."

Grenville Clark was born on November 5, 1882, in New York City. His parents were Louis Crawford Clark and Marian de Forest Cannon. His father attended Harvard University and worked all his life at his father's banking firm (Clark, Dodge & Company) on Wall Street. His mother was the granddaughter of banker Colonel LeGrand Bouton Cannon (1815–1906); the family lived in her grandfather's mansion in Manhattan. He had three brothers and a sister.

In 1903, he entered Harvard Law School, where he met classmate Felix Frankfurter. "The two became close friend and sometimes conspirators on public service projects throughout their lives." The childless Frankfurter was unofficial "uncle" to Clark's children. When Frankfurter retired from the United States Supreme Court, Clark took care of financial matters like medical bills.

In 1909, Harvard Law School graduates Clark, Francis W. Bird, and Elihu Root, Jr. (son of Elihu Root, Sr.) founded the law firm of Root, Clark & Bird. An early member and star of the firm was Henry Friendly, recommended by Frankfurter and following a one-year clerkship with Supreme Court Justice Louis Brandeis. In 1913, the firm merged with Buckner & Howland to form Root, Clark, Buckner & Howland. In 1919, Arthur A. Ballantine, the Internal Revenue Service's first solicitor, joined. Ballantine and Buckner ran the firm throughout the 1920s and 1930s. During the Great Depression, the firm flourished by moving away from its traditional focus on litigation and began to focus on bankruptcy and reorganizations and New Deal regulations. The firm also built up a corporate practice, serving clients such as AT&T and Standard Oil. Overall, the firm expanded from eight (8) to 74 associates and opened a second office in Washington, D.C. Both Henry Friendly and John Marshall Harlan II worked at the firm during this period. In 1931, the offices of Root, Clark, Buckner & Ballantine were at 31 Nassau Street (Manhattan) in Manhattan. (The firm later merged to form Dewey Ballantine, currently Dewey & LeBoeuf.)

During World War I, Clark helped establish the Citizens' Military Training Camp at Plattsburgh, New York and other forms of military preparedness.

In 1931, Clark was elected one of the five fellows (plus Harvard University president and treasurer) of the Harvard Corporation, a body overseeing the university since 1650. In 1933, he was instrumental in having James Bryant Conant elected president of Harvard University; the two become close friends. In 1950, he retired from the Corporation, for which he received an honorary doctoral degree in law.

In 1933, he helped draft Economy Act of 1933. He joined in drafting committee amicus briefs to the Supreme Court in defense of free speech and, in the flag salute cases, asserting the right of individuals on conscientious grounds not to salute the flag. In July 1938, Clark founded the American Bar Association’s Bill of Rights Committee, as a means to unite both left and right in defense of civil liberties. As illustrative examples of increasing threats to individual freedom, he cited “suppression of free speech and assembly” by Jersey City Mayor Frank Hague, attacks on the people's “right of petition concerning legislation” by Senator Sherman Minton and, finally, proposals “to establish Governmental radio broadcasting.”