[4] Testa made her debut Off-Broadway at Playwrights Horizons as Miss Goldberg in William Finn's one-act musical In Trousers (1979), part one of his "Marvin Trilogy.

[2][4] Her Broadway roles include Joyce Heth in Barnum (1982), movie columnist Hedda Hopper in Marilyn: An American Fable (1983), Angel in The Rink (1984), Domina in A Funny Thing Happened on the Way to the Forum (1996–1997), Madame Dilly in On the Town (1998), Magdalena in Marie Christine, Maggie Jones in 42nd Street (2001–2002), the Matron in Chicago (2005), Melpomene in Xanadu (2007–2008), General Matilda B. Cartwright in Guys and Dolls (2009), Madame Morrible in Wicked (2014), and Aunt Eller in Oklahoma!

[5] She is a frequent collaborator with such acclaimed musical dramatists as William Finn (Infinite Joy, A New Brain, In Trousers) and Michael John LaChiusa (Marie Christine, See What I Wanna See, First Lady Suite), while also having appeared in the works of Stephen Sondheim, Kander & Ebb, Flaherty & Ahrens, and Leonard Bernstein, among other Off-Broadway and regional theatre credits.

[6] On December 28, 2020, it was announced that Testa would star as Skinner in a benefit concert presentation of Ratatouille the Musical, an internet meme that originated on TikTok, inspired by the 2007 Disney/Pixar film.

She also appeared in such films as The Business of Strangers (2001), Stay (2005), Eat Pray Love (2010), The Bounty Hunter (2010) and the TV series Law & Order, Whoopi's Littleburg, 2 Broke Girls, Smash, White Collar, Cosby, Sex and the City, and Life on Mars.