The 69-year-old star had performed the legendary Mattel character on more than 100 talking Barbie games, toys, dolls and picture books from 1994 until 2006.
"I was not allowed to tell anybody I was doing Barbie's voice, the whole time," Lansdowne explained in a recent appearance on the podcast Toon'd In! with Jim Cummings.
"They did not want Barbie to have a face, a person. They wanted the mystique. I could not tell people I was doing Barbie for all those years. That was hard!
"And she was talking and everything. Everybody wanted interviews. It was like the height of her life."
The star kept it a secret until the release of Toy Story 2 in 1999 - but Disney chose Jodi Benson, 64, to voice Barbie in the film because they were unaware Lansdowne was already the doll's voice actor due to the secrecy surrounding the role.
"I couldn't tell until Toy Story came out. They said, 'You're going to do Toy Story.' And I went, 'OK, this is tricky.'
"Disney didn't even know about me. So they went to somebody else. And I went, 'Now the credits will roll. They'll see that name. They're going to go, 'Oh, that's who Barbie is.'"
Despite not playing the role in Toy Story 2, Lansdowne feels lucky to have voiced Barbie for many years.
"But you know what? All I can say is this. I am more than grateful that I got to do that voice, period. I mean, who's going to complain?
"I never did negative things. I kept her clean and lovely, and that's how I was the Barbie of the '90s.
"I had to tell you that. I couldn't tell anybody, but I called kids on the phone and went in hospitals all the time ... Mattel never knew, now they know."
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