Born to a prominent family, Seymour graduated from Princeton University and Yale Law School and served in the United States Army during World War II.
In 1986, he was appointed as independent counsel to investigate former Reagan White House official Michael Deaver, and successfully secured a perjury conviction the next year.
As a civic leader in New York, he served on a number of boards, and played an important role in the Municipal Art Society's push for passage of the city's 1965 Landmarks Law.
[4] Seymour's father was a prominent attorney[4] who served as assistant solicitor general during the Herbert Hoover administration.
[1] He joined the U.S. Army in 1943,[6] serving as an artillery officer in the Pacific theater during World War II, and resigning in 1945 with the rank of captain.
[1] Seymour then returned to private practice before being appointed, three years later, as counsel to the State Commission on New York City Governmental Operations.
[12][13] The NRDC's establishment was partially an outgrowth of the Scenic Hudson Preservation Conference v. Federal Power Commission, the Storm King case, in which Seymour was involved.
[12] The case centered on Con Ed's plan to build the world's largest hydroelectric facility at Storm King Mountain.
The proposed facility would pump vast amounts of water from the Hudson River to a reservoir, and release it through turbines to generate electricity at peak demand.
[14] Realizing that continued environmental litigation would require a nationally organized, professionalized group of lawyers and scientists, Duggan, Seymour, and Sive obtained funding from the Ford Foundation[12][14] and joined forces with Gus Speth and other recent Yale Law School graduates of the class of 1969 to form the NRDC, with John H. Adams as the group's first staff member, Duggan as its first chairman, and Seymour, Laurance Rockefeller, and others as board members.
[15][16][a] As U.S. Attorney, Seymour also prosecuted New York City Police Department corruption and misconduct cases brought by the Knapp Commission.
Under Seymour, former Richard Nixon Cabinet members John N. Mitchell and Maurice H. Stans were indicted on charges of accepting illegal campaign contributions from fugitive Robert Vesco, but both were acquitted.
"[2] Later, however, Seymour was critical of the Times's handling of the case; in a 1994 article in the New York State Bar Journal, he wrote that he remained "appalled at the arrogance and irresponsibility displayed by the news media in setting up a totally unnecessary confrontation over publication of stolen classified documents relating to U.S. policies in Vietnam.
"[19] In Seymour's view, from a practical perspective, the government had "lost the battle but won the war" in the Pentagon Papers cases, since the Times and Washington Post, following the Supreme Court's decision, did not publish material whose release could damage national security, such as the "secret Defense Department study directly affecting military and intelligence operations and secret diplomatic efforts to achieve peace.
[2][24] He joined with another lawyer, Peter Megargee Brown (formerly of Cadwalader, Wickersham & Taft), to form a small two-person firm.
[24][25] In May 1986, a panel of three federal judges appointed Seymour as independent counsel to investigate Michael Deaver, a senior aide to President Ronald Reagan.
[27] Deaver was indicted on five counts of perjury on charges that he had given false testimony to a grand jury that he did not remember a January 1985 meeting with Canadian ambassador Allan Gotlieb and his wife Sondra.
[1] In August 1964, the Municipal Art Society designated Seymour as the leader of its efforts to permanently establish the New York City Landmarks Preservation Commission.
[38] Seymour was a staunch opponent of political action committees, believing them to have a malign effect on Congress, and was a founder of Citizens Against PACs.