She began writing the "Ask Ann Landers" column in 1955 and continued for 47 years, by which time its readership was 90 million people.
A 1978 World Almanac survey named her the most influential woman in the United States.
She was the identical twin sister of Pauline Phillips, who wrote the "Dear Abby" advice column as Abigail Van Buren.
Lederer was a profile-raiser for several medical charities, and in 1977 President Carter appointed her to a six-year term on a cancer advisory board.