No Biography, But Look:
Apparently, she "loathed playing piano for people; always have," but was quite good at it and made several records.
Piano Prodigy Played Sassy Sisters
I had no idea Diana Lynn played piano at all, let alone that her virtuosity is what got her into pictures in the first place. Play she did, and in her first two pictures, the pre-teen was billed at "Dolly" Loehr until 1942, when Paramount signed her to a long-term contract and changed her name to "Diana Lynn."
Lynn's first feature role was as Ray Milland's fiancee (Rita Johnson)'s kid sister in The Major and the Minor (1942), and while she's not in it for long, she's pivotal -- wise, canny, and helpful. Similarly, in The Miracle of Morgan's Creek, Lynn plays a smarter, infinitely more mature kid sister to poor ol' Betty Hutton.
As with many other child actors that preceded her, Diana Lynn got fewer and roles as she got older. It's not a little confounding, because she played such sophisticated, mature kids, but what do I know? In the 1950s, she worked mainly on television, but did have an adult lead opposite Ronald Reagan in Bedtime for Bonzo (1951), which may very well have been the last straw. She also performed on Broadway and on the London stage.
Her first marriage was in 1948 to architect John C. Lindsay, whom she divorced in 1953. Several years later, she married Mortimer Hall, son of Dorothy Schiff, publisher of the New York Post. Lynn felt that it was possible to balance career and family, and when interviewed in the 1960s about how she managed it all, she said, "I don't burden my husband with every detail of stagecraft...[and] I try to be instinctive about raising my children. I try to hear what they're not saying. It's working out. They're nice; they're happy; they've got manners."
She had been running a fledgling travel agency in New York City when Paramount offered her the lead role in the film adaptation of Joan Didion's novel, Play It as It Lays. Just before shooting began, Lynn had a fatal stroke and died at the age of 45 on December 18, 1971. She left behind her husband and four children, two of whom, Dolly and Daisy, are in show business today.
I recommend Miracle of Morgan's Creek for a happy example of this fine actress' work.