The son of Harry and Nan Hight McPherson, he attended Southern Methodist University and graduated from the University of the South in 1949. He intended to be a writer and poet and enrolled in a graduate program in literature at Columbia in 1949. But when the Korean War broke out in 1950, he enlisted in the Air Force and served as an intelligence officer in Germany, assessing Soviet troop deployments. His first marriage, to Clayton Read in 1952, ended in divorce. He married Mary Patricia DeGroot in 1981. Besides his wife, he is survived by two children from his first marriage, Coco and Peter, and a son from his second marriage, Samuel. He earned a law degree in 1956 at the University of Texas. At the urging of a cousin who worked for Johnson, who was then a senator, Mr. McPherson went to Washington and was hired by the Democratic Policy Committee, which fashioned the legislative agenda for Senate Democrats. Johnson, the majority leader, was its chairman. Over the next seven years, Mr. McPherson rose to general counsel of the committee, serving under Senator Mike Mansfield of Montana after Johnson became vice president in 1961. He was appointed deputy under secretary of the Army for international affairs in 1963 and assistant secretary of state for educational and cultural affairs in 1964. Nearly two years after the assassination of President John F. Kennedy, Mr. McPherson joined President Johnson’s White House staff in 1965, and over the next four years, he became one of Johnson’s most trusted advisers. In 1966 he helped organize a White House conference on civil rights, a gathering that included the Rev. Dr. Martin Luther King Jr.; Thurgood Marshall, who was then the solicitor general but would become America’s first black Supreme Court justice a year later; and representatives of almost every major civil rights group in the country. Mr. McPherson also became Johnson’s chief speechwriter, shaping all of the president’s major addresses from 1966 to 1969. After leaving the White House in early 1969, Mr. McPherson became a partner in Verner, Liipfert & Bernhard, a prominent Washington law firm and one of the capital’s most successful at lobbying. His clients included businesses, foreign governments and nonprofit organizations.