The hero, David Harding, was portrayed by a lot of different actors but Don MacLaughlin lasted the longest; others were Phil Sterling, Lawson Zerbe and House Jameson. Although Harding was the central protagonist at the start of the series run, by post-war he always accompanied by agent Harry Peters (Mandel Kramer; also Lawson Zerbe and Phil Sterling), who did most of the undercover work on the show.
The series was produced by Phillips H. Lord, best-known as the creator of Gang Busters but also creator of Mr. District Attorney. Lord was apparently inspired by current events, looking to the USA's entry into World War II as an opportunity to tell espionage tales featuring Nazi spies.
Strangely, Counterspy became less of a spy program after the war ended. While the wartime version of the series featured plenty of Axis agents, you'd expect they'd turn to communist agents post-war; and yet, the post-war episodes I've heard were concerned primarily with breaking up crime rackets, resulting in a show that sounds almost identical to Lord's Gang Busters. When the series moved to NBC, communists did indeed pop up as adversaries, but Counterspy's agents seemed mostly interested in breaking up criminal gangs. They're very well-known to the public too, who are constantly exclaiming, "It's the Counterspies!"
Thanks to the show's twice-a-week format during the Pepsi-Cola years there were even a few two-part Counterspy episodes, like the story titled "The Case of the Cold-Blooded Professor" (August 30/September 1, 1949) in which the first part detailed the capture of a murderer, then saw him escape in part two, resulting in a pursuit to recapture him. Another two-parter was "The Case of the Postal Pirates" (September 20 and 22, 1949).
As the show went on, I noticed the Pepsi-Cola advertisements were integrated with the plot. Very frequently, Harding and Peters would specifically pause to enjoy a cool, refreshing Pepsi-Cola and comment to each other what a fine beverage it was. If the writers thought this was subtle they were mistaken.
Counterspy isn't bad-- those earliest episodes where Harding would recruit ordinary people as spies against the Axis is very different from other espionage radio shows. However, as the show went on, I felt it became a bit more juvenile (maybe it's just those cheerful Pepsi-Cola jingles). It was never a very sophisticated program but those earliest broadcasts are interesting as a wartime spy programmer.
Counterspy spawned two movies: David Harding, Counterspy (1950) and Counterspy Meets Scotland Yard (1950), both starring Howard St. John as David Harding.
The Old Time Radio Researchers have a YouTube playlist of Counterspy.