MacFadyen was born Gavin Hall Galter on January 1, 1940, in Greeley in Colorado, and grew up in Chicago. His mother was a pianist. He adopted the surname of his stepfather, a medical researcher. He studied at Shimer College from 1958 to 1959, and later worked as a union organizer with trade unions. He was jailed for participating in civil rights demonstrations. He moved to England, joined the International Socialists (which later became the Socialist Workers Party (UK)), and graduated from the London School of Film Technique (now the London Film School). He founded a documentary film group to chronicle the political turmoil in the United States during the late 1960s for the BBC. He covered the anti-Vietnam War protests, race riots and the police clash with demonstrators at the Democratic National Convention in Chicago in 1968, where tens of thousands of Vietnam War protesters battled police in the streets, while the Democratic Party fell apart over an internal disagreement concerning its stance on Vietnam. He went on to report from the Nicaraguan Revolution, a war between the right-wing Contras and the Marxist Sandinista National Liberation Front in the 1980s, and Iran-Contra, neo-Nazi violence, Watergate, the history of CIA.