For three years starting in 1946, he, along with Byrum Saam and Claude Haring, called all the home games of both Philadelphia professional baseball teams, the Athletics and Phillies.
In 1949, Thompson was hired by the Gunther Brewing Company to be WITH's play-by-play voice for both the International League Orioles and the Colts, at the time a member of the All-America Football Conference (AAFC).
When the American League's St. Louis Browns moved to Baltimore, Maryland, and were rechristened the Orioles in 1954, his previous connections with Gunther prevented him from becoming a broadcaster for the franchise.
The National Brewing Company had purchased the team's broadcast rights and hired Ernie Harwell as the lead voice, but still wanted Thompson to be part of the coverage.
Two years later he joined Bob Wolff to call Washington Senators games on WWDC and WTOP-TV, succeeding Arch McDonald as a result of National Brewing's becoming the team's new sponsor.
Others who worked with Thompson on Orioles broadcasts included Frank Messer (1964–1967), Jim Karvellas (1968–1969), John Gordon (1970–1972), Brooks Robinson (1978–1987), and Tom Marr (1979–1982).
Besides his baseball-related achievements, Thompson also called Colts football for many years, first on CBS television in the 1950s and '60s, and then alongside Vince Bagli on WCBM Radio from 1973 until the team's relocation to Indianapolis in 1984.
In 1988, he was among several veteran announcers who called some September NFL telecasts for NBC while many of the network's regular broadcasters were working that year's Summer Olympics in Seoul, South Korea.
He is particularly remembered for his flawed but endearing call of Bill Mazeroski's championship-clinching home run to end the 1960 World Series, for which he was the play-by-play announcer for NBC Radio.
The pitcher was actually Ralph Terry; Art Ditmar was warming up in the bullpen, and besides that error, Thompson just got caught up in the moment: Well, a little while ago, when we mentioned that this one, in typical fashion, was going right to the wire, little did we know…Art Ditmar throws—here's a swing and a high fly ball going deep to left, this may do it!…Back to the wall goes Berra, it is…over the fence, home run, the Pirates win!…(long pause for crowd noise)…Ladies and gentlemen, Mazeroski has hit a one-nothing pitch over the left field fence at Forbes Field to win the 1960 World Series for the Pittsburgh Pirates by a score of ten to nothing!…Once again, that final score…The Pittsburgh Pirates, the 1960 world champions, defeat the New York Yankees.
and Forbes Field... is an insane asylum!In 1985, Thompson's Ditmar-Terry flub became a commercial hit, featured as an audio-over in a nostalgia-immersed Budweiser TV ad during that year's World Series.
A libel lawsuit subsequently filed by Ditmar against Anheuser-Busch and its advertising agency for the commercial was ultimately rejected by a United States District Court.