Catherine (Cassie) Patricia Mackin was born 28 August 1939 in Baltimore, Maryland, to Francis and Catherine Mackin. Raised in Baltimore, she attended a local parochial school. Winning a four-year scholarship, she went to the Institute of Notre Dame before entering Washington College in 1956. A year later she transferred to the University of Maryland at College Park. While a student there, she worked for the now defunct Free State Press, a weekly paper published in suburban Washington, D.C. She graduated magna cum laude in June 1960 with a B.A. degree in English and minors in economics and history.
After graduation, Mackin obtained a position at the News American in Baltimore. Beginning as a general assignment reporter, she held a variety of positions before she left the paper in 1963. Between 1960 and 1962 she made guest appearances on both a Baltimore news panel show and on a morning variety show. From 1963 until 1969, Mackin was employed by the Hearst newspapers in their Washington Bureau. During the six years she worked for Hearst, she covered the Justice Department, numerous elections, and presidential campaigns, polishing the talents that would make her a successful national news correspondent.
In 1967, Mackin became one of the few women to receive a Neiman Fellowship to Harvard University where she studied the history of political institutions. Hired by NBC in 1969, she anchored a half hour newscast at WRC TV, the Washington affiliate of NBC, in addition to her reporting responsibilities. A tough minded and tenacious interviewer, Cassie Mackin received national attention three years later when she became NBC's first woman floor reporter at the Democratic and Republican presidential conventions. Her work contributed to the subsequent award of an Emmy to the NBC news team for its coverage. After a brief stint in Los Angeles, Mackin returned to Washington D.C., to become the Sunday evening anchor and congressional correspondent in 1974.
In September 1977, Mackin joined ABC as their Washington correspondent, temporarily covering the Senate. She also worked on a 20/20 story about drunk driving, for which she received another Emmy in 1981. Assigned to the 1980 presidential campaign, Mackin spent a good portion of the year following Senator Edward Kennedy around the country.
Mackin moved to Baltimore in October 198l to live with her sister, Margaret, after having a kidney removed in an effort to stop the spread of cancer. She appeared periodically on "This Week with David Brinkley" as part of a discussion group and updated her drunken driving report for 20/2020/20", but succumbed to cancer on 20 November 1982 at the age of 43.