Marlo Thomas never did another series after That Girl ended its run in 1971 after 5 seasons. Today, she carries on the work of her father, Danny, with St. Jude's Children's Hospital in Memphis.
Father & daughter would appear together in two episodes of That Girl, but only once would they sing together. I don't know the name of the song used here in the episode, "The Friars", but Danny appears as himself attempting to buy an old trunk from Ann Marie (Marlo). Milton Berle also makes a bid, and ends up trapped in the trunk. He's heard from, too, in the following clip. If anyone can tell me the name of the song (also performed by Maureen McCormick & Florence Henderson on The Brady Bunch), please contribute to the comments section. Thanks.
Oh, by the way, I'm not sure if Marlo herself or a fan using her name as a screen name has the YouTube channel from whence we get this gem.