Born as Ruth Maxine Kahn in Des Moines, Iowa, Kobart studied opera at the American Conservatory of Music in Chicago and made her professional debut as the Witch in an off-Broadway production of Engelbert Humperdinck's Hänsel und Gretel.

Additional Broadway credits included How to Succeed in Business Without Really Trying, A Funny Thing Happened on the Way to the Forum, A Flea in Her Ear, and Three Sisters.

Her appearances with them included The House of Bernarda Alba, Sunday in the Park with George, Arsenic and Old Lace, A Little Night Music, and Home.

In the 1970s she took an extended leave from the company to portray Nurse Ratched in the 18-month-long San Francisco production of One Flew Over the Cuckoo's Nest.

Her portrayal of Miss Hannigan on the first tour of Annie and in a long Chicago run (in succession to originator Dorothy Loudon) was considered one of the best interpretations of that classic role.