The young Lockhart first registered in movies as the older brother's charming girlfriend whom Judy Garland initially misjudges in Vincente Minnelli's classic "Meet Me in St. Louis" (1944). Her future beckoned as she played second fiddle to both a remarkable canine and her leading man (Peter Lawford) in the rousing WWII-era sequel "Son of Lassie" (1945). In her first starring role, Lockhart acquitted herself well as a woman manipulated to think herself the "She Wolf of London" (1946), a minor but mildly diverting thriller. She also provided sturdy support to a crime-fighting hubby in "T-Men" (1947), an exceedingly stylish noir outing from director Anthony Mann.

After frequent TV appearances as a dramatic anthology player, game show panelist and "women's show" guest hostess, Lockhart made her first serious bid for pop culture immortality by joining the long-running (CBS, 1954-1971) children's classic "Lassie" in 1958. As Ruth Martin, she was half of a childless couple that adopts the lovable runaway orphan Timmy (Jon Provost) and the courageous collie. Lockhart helped guide the duo's adventures for six years, garnering a 1958/59 Emmy nomination for Best Actress in a Continuing Performance in a Dramatic Series. She had plentiful adventures of her own as the mother of the Space Family Robinson in the campy sci-fi classic "Lost in Space". Her Maureen Robinson managed to preserve her family's values in the most outlandish situations with a reassuring word, a smile and a slice of her "space pie". She also displayed a wholesome devotion to her man, the hunky Professor Robinson (Guy Williams), and a remarkable tolerance for the self-centered Dr. Smith (Jonathan Harris). Lockhart finished out the 60s, playing "a lady M.D.", Dr. Janet Craig, on the last two seasons of gently rustic sitcom hit "Petticoat Junction" (CBS, 1968-70). She also began long stints as a hostess on several major beauty pageants and parades covered by CBS.

Lockhart spent the 70s in TV-movies, miniseries, a children's series and some busted pilots. The 80s brought more of the same, as well as stints on daytime and primetime soaps, notably playing Maria Ramirez, a kindly matriarch on ABC's daytime drama "General Hospital". Lockhart began popping up in small feature roles, generally in genre outings, that played upon her wholesome maternal image. She was the mother of scientist Paul Le Mat in "Strange Invaders" (1983), an engaging tribute to 50s sci-fi. "Troll" (1986), a passable knock-off of 1984's "Gremlins", found her playing a heroic witch who battles trolls in her enchanted apartment building. Lockhart also provided colorful cameos for "The Big Picture" and "CHUD II: Bud the Chud" (both 1989). She continued to work on stage, even touring nationally with "Steel Magnolias" from 1989 to 1990.

Lockhart has remained busy in the 90s, appearing on TV periodically in guest shots ("Roseanne"; "The Critic"; "Step By Step") and nostalgic specials. Her only feature credit in the first half of the 90s was a small part in the indie romantic drama "Sleep With Me" (1994). Lockhart has worked as a corporate spokesman for several organizations and has served on numerous committees and boards.