Dart, who was also an arts patron and philanthropist, died Wednesday at her home in Pebble Beach, Calif., after a lengthy illness, her family said.
Warner Bros. renamed her Jane Bryan after signing her to a contract around 1936 and often cast her as the ingenue. Bette Davis took the actress under her wing, and Dart was "far more effective in roles calling for disillusionment in pathos," notably as Davis' sister in 1937's "Marked Woman," wrote Hal Erickson in the "All Movie Guide."
She gave her finest performance in "We Are Not Alone" (1939) as the doomed mistress of Paul Muni, Erickson wrote.
With Reagan and his first wife, Jane Wyman, she appeared in 1938's "Brother Rat" and "Brother Rat and a Baby," the 1940 film that was her last.
On New Year's Eve 1939, she married Dart, who would take over the floundering Rexall Drug chain in 1945 and build it into Dart-Kraft Inc., a food and consumer products conglomerate. He died in 1984 at 76.
When she walked away from Warner Bros. to wed, The Times' headline on the 1939 story declared: "Love Triumphs Over Career." She had appeared in almost 20 films.
The Darts lived in Chicago and Boston before moving to Los Angeles in the mid-1940s and becoming involved in civic affairs and charity work, her family said. The couple built a house in Bel-Air and had a weekend home near Palm Springs.
They soon began having dinner with Reagan and Wyman.
"At the time he was a rabid Democrat," Justin Dart recalled in 1980. "My wife warned me not to talk politics."
After Reagan married his second wife, Nancy, in 1952, the Darts remained part of Reagan's inner circle. The former first lady came to appreciate Jane's "discretion and independence," Bob Colacello wrote in the 2004 biography "Ronnie & Nancy."
"Jane Dart was an old and very dear friend, and I was sorry to hear of her passing," Nancy Reagan said Friday through a spokeswoman.
Justin Dart was among a handful of wealthy Southern California businessmen who helped persuade Reagan to enter politics and then run for governor in 1966.
The Darts amassed a collection of about 70 artworks that Jane donated to the Monterey Museum of Art. They are housed in the Jane and Justin Dart Wing in La Mirada, a satellite facility of the Monterey museum, where she served as a trustee.
A native of Los Angeles, Jane O'Brien was born June 11, 1918, to James Matthew O'Brien, a lawyer, and his wife, the former Irene Murray.
Justin Dart was known to be blunt and outspoken, but his wife was "shy, reserved, self-effacing," Colacello wrote.
She had a keen interest in archaeology and Egyptology and liked to be called by the nickname her husband gave her -- "Punky."
Dart is survived by her three children, Guy Michael Dart of Los Angeles, Jane Tucker of Dallas and Stephen M. Dart of Pebble Beach; brothers Donald O'Brien of Denver and William O'Brien of Paradise Valley, Ariz.; three grandchildren; and two great-grandchildren.
Services will be held at 4 p.m. Friday at the Church in the Forest, 3152 Forest Lake Road, Pebble Beach.
Memorial donations may be made to the charity of the giver's choice or to the Monterey Museum of Art, 559 Pacific St., Monterey, CA 93940.