Lockhart is best known for her roles as TV mothers, first as Ruth Martin, the wife of Paul Martin
(portrayed by Hugh Reilly) and the mother of Timmy Martin (played by Jon Provost) in the 1950's
CBS series Lassie (a role that she played from 1958 to 1964). She replaced actress Cloris Leachman,
who, in turn, had replaced Jan Clayton who had played a similar character earlier in the series.
Following her five-year run on Lassie, Lockhart made a guest appearance on Perry Mason as defendant
Mona Stanton Harvey in "The Case of the Scandalous Sculptor". Lockhart then starred as
Dr. Maureen Robinson in Lost in Space, which ran from 1965 to 1968 on CBS, opposite veteran actors
Guy Williams and Jonathan Harris.