It was signed on by Don Lee Broadcasting, which owned a chain of radio stations on the Pacific coast, and was first licensed by the Federal Radio Commission (FRC), forerunner of the Federal Communications Commission (FCC), as experimental television station W6XAO in June 1931. The station went on the air on December 23, 1931, and by March 1933 was broadcasting programming one hour each day on Mondays through Saturdays. The station used a mechanical camera, which broadcast only film footage in an 80-line image, but used all-electronic receivers as early as 1932. It went off the air in 1935, and then reappeared using an improved mechanical camera producing a 300-line image in June 1936. By August 1937, W6XAO had programming six days each week, with live programming starting in April 1938.
On May 6, 1948, the station changed its call sign to KTSL and was named for Thomas S. Lee, the son of Don Lee. The station became affiliated with the DuMont Television Network later that year.
Don Lee's broadcasting interests were placed for sale in 1950 following the death of Thomas S. Lee. General Tire and Rubber agreed to purchase all of Don Lee's stations, the centerpiece being KHJ radio, but chose to spin-off KTSL to CBS. Subsequently, CBS sold its share in KTTV to the station's majority partner, the Los Angeles Times, and all CBS programming moved to KTSL on January 1, 1951. On October 28, 1951, KTSL changed its callsign to KNXT (presumably meaning "KNX Television") to coincide with CBS' Los Angeles radio outlet, KNX (1070 AM). The station also moved its transmitter from Mount Lee, where it had been based since its experimental days, to Mount Wilson.
The logo design used by KCBS-TV during this time period also served as a template for logos used by KREM in Spokane, Washington, KTVT in Fort Worth-Dallas, and KSTW in Tacoma-Seattle used around the same time.
Since 2016, this logo is also being used by East Coast flagship sister station WCBS-TV in New York City.