Reasoner was born at First Street North in Dakota City, Iowa; he and his older sister, Esther, were the children of Eunice (Nicholl) and Harry Ray Reasoner, who married in 1911. Reasoner was taught to read by his parents before entering school, gaining a strong vocabulary from his mother. Reasoner attended West High School in Minneapolis. During his time at the school, Reasoner developed his interest in journalism.

In 1970, Reasoner was hired away from CBS by ABC to become an anchor on the network's newly revamped nightly newscast. At the time of his hire, the network's New York-based broadcast, ABC News, was anchored by Howard K. Smith and Frank Reynolds, both former colleagues of Reasoner at CBS. Beginning in December 1970, Reasoner was moved into Reynolds' position (Reynolds thus became the network's chief Washington correspondent), and the newscast became known as ABC Evening News.  Reasoner anchored the news alongside Smith until 1975, when he took the sole anchor position while Smith moved into a commentary role. The next year, however, ABC decided to pair Reasoner with a new co-anchor, former Today Show co-host Barbara Walters; ABC had gone to great lengths to hire her away from NBC. Walters and Reasoner did not enjoy a close relationship; Reasoner did not like sharing the spotlight with a co-anchor and also was uncomfortable with Walters' celebrity status. It was also widely believed that Reasoner disliked the idea of a woman anchoring the network news, which he himself denied: "I am trying to keep an open mind about it." In another interview, Reasoner said, "I've worked in journalism for women and with women for years. For two years I did a CBS morning news program with a woman. I feel they're no worse than men are."  After two years of co-anchoring ABC Evening News with Walters, Reasoner departed the network after nearly eight years in June 1978 and returned to CBS, where he resumed his duties on 60 Minutes. Shortly after his departure ABC elected to scrap ABC Evening News altogether and reworked the newscast into World News Tonight.  Reasoner stayed with 60 Minutes until his retirement, on May 19, 1991.

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