The 76-year-old said she was "thrilled" to be playing Carol Tregorran, a role originally played by Anne Cullen.
Listeners learned Carol had returned to Ambridge to attend the funeral of her husband John in Friday's episode.
Sean O'Connor, editor of The Archers, said Bron "fitted [in] as if she has been here for 30 years".
"In the 1950s, Carol was a significant role model for young women," continued O'Connor, a former EastEnders producer who took over his Archers role last September.
"We hope she will continue to be so for older women today - an independent, spirited woman who embraces her later years on her own terms and without compromise."
Carol Grey made her first appearance in The Archers in 1954 and married John Tregorran in 1967. She and John left Ambridge in 1990, returning for special occasions in 2007 and 2010.
Her place in Archers history is assured, having been dining at Grey Gables on the famous night in 1955 when Grace Archer was killed in a fire.
Bron, who began her career with the Cambridge Footlights in 1959 and appeared in such classic 1960s films as Help! and Alfie, described Carol as "quite forceful and independent, even now in her 80s".
"Now she has returned to Ambridge she is able to be around friends," she went on. "There's something very wonderful when you can pick up those sorts of friendships and see them play out in a drama."
Eleanor Bron has joined the cast of Radio 4's rural drama series The Archers, playing a character who first appeared in it 60 years ago.