Mary Livingstone, the widow of Jack Benny, who for decades on radio and television served as her husband's comedic foil, died after a short illness Thursday at her home in Holmby Hills, near Los Angeles. She was 77 years old.

For nearly 50 years Miss Livingstone was married to the taciturn comedian, who was famous for his supposed stinginess and bad violin playing.

She was one of the fixtures of her husband's radio show, which for 21 years was a Sunday night must for millions of Americans. She played his wisecracking partner as part of a cast of characters that included the boyish Dennis Day, the skeptical, gravel-voiced valet Rochester (Eddie Anderson), the ebullient Don Wilson and the bantering Phil Harris.

In the years after Mr. Benny's death in 1974, Miss Livingstone wrote remembrances of him for magazines. In 1978, with her brother Hilliard Marks (a longtime writer and producer for Mr. Benny) and Marcia Borie, Miss Livingstone wrote an affectionate biography about a man who was almost the opposite of his stage character. 'One Long-Stemmed Rose'

''Every day since Jack has gone the florist has delivered one longstemmed red rose to my home,'' she wrote in a magazine article. ''I learned Jack actually had included a provision for the flowers in his will. One red rose to be delivered to me every day for the rest of my life.''

Mary Livingstone Benny was born in Seattle to a couple called Marks and was named Sadie, but she later had her name legally changed. According to a 1942 publicity release, Miss Livingstone was 12 years old when she first met Jack Benny, who was calling on her oldest sister.

Several years later, they met again at the May Company store in downtown Los Angeles, where she sold women's hosiery. Two weeks of dates were followed by two years of separation. But when they met for a third time, in 1927 in Chicago, they decided to marry despite Miss Livingstone's distaste for the road life associated with vaudeville. Several years later they adopted a daughter, Joan.

Their meeting at the May store became a standard part of their act. ''I didn't mind,'' she once said. ''After all, the little girl from hosiery and the guy who picked her up there had just bought Tom May's former house in Palm Springs.'' Stepped Into the Role

Soon after they were married, the young woman who played opposite Mr. Benny became ill while he was performing in New York City and Miss Livingstone stepped into the role.

It was temporary that time. But later, while the show was in Chicago, Mr. Benny's new partner left and Miss Livingstone replaced her until the tour reached Los Angeles. There, Mr. Benny tried one more candidate before asking Miss Livingstone to be his partner on stage as well as in real life.

When Mr. Benny started on NBC in 1932 he worked alone until one night the script ran short and he got Miss Livingston to join him in a bit of ad-libbing to fill out the program. The result was so popular with listeners that she became a permanent part of the act.

During their heyday they were among radio's reigning families, which also included their good friends George Burns and Gracie Allen. Miss Livingstone tried one excursion into movies in 1937. She was featured in ''This Way Please,'' with Fibber McGee and Molly, another comic radio couple.

For years she had been a reluctant member of her husband's act and, except for a few televison appearances, she retired from show business in the 1960's. She remained, however, a popular Hollywood hostess.

She is survived by her daughter, Joan Naomi Blumofe, and four grandchildren. Services will be held tomorrow.

Mary Benny