Donaldson broke into broadcasting soon after his discharge from the US Army in 1959 and within a year had gone from being a general announcer at a station in Dallas to first a general reporter then anchor at WTOP in Washington, DC. He remained at WTOP for six years before ABC lured him away, naming him as a general reporter and later the Capitol Hill correspondent (1968-77). Donaldson moved to covering The White House until the end of the Reagan administration in 1989. At the same time, he departed from his decade-long stint anchoring the ABC Weekend News and became co-anchor (with Diane Sawyer) of "PrimeTime Live" (1989- ). He also was a regular interviewer and commentator on Sunday morning's "This Week With David Brinkley".
Through his decades with ABC, Donaldson has covered several political conventions and campaigns and his 1975 primetime special, "ABC News Close-up on Washington Regulators: How They Cost You Money" was ground-breaking in its time. Donaldson was the first US network TV reporter to provide live coverage from Saudi Arabia after the Iraqi invasion of Kuwait in 1990, and coverage from Panama after the US invasion in 1989. For "PrimeTime Live", Donaldson has tracked down Nazi war criminal Erich Priebke and, along with Sawyer, did an unprecedented broadcast from inside the Kremlin in 1989. But he is perhaps best known for his coverage of the Washington political scene and climate. His experience in that realm dates back to 1964 when he followed the presidential campaign of Barry Goldwater.