Botany Mills, in 1941. That was the year when the government first allowed a TV network to charge for TV ads. Botany Mills, a textile manufacturer, was the only sponsor known to produce ads on film that year, and the company was the first to create animated ads made specially for television. Douglas Leigh was hired to make seven animated ads, which starred the Botany Lamb character. The lamb would first promote the Botany Mills' line of wool ties, then look into a telescope to predict tomorrow's weather.
Karl Cohen's full article, "The Development of Animated TV Commercials in the 1940s," can be found here: https://web.archive.org/web/20030102082632/http://www.chapman.edu/animation/karl.html