Marianne L. Lopat, 28, a former legislative assistant on Capitol Hill, died of cancer Sunday at the South Coast Medical Center in Laguna Beach, Calif.

She came to Washington in 1974 to work for Rep. James R. Mann (R-S.C.). Later she worked for Sen. Peter Dominick (r-Colo.) and in the Washington office of Gov. Hugh Carey of New York.

Miss Lopat then was a legislative assisant to Rep. John Anderson (R-Ill.) until leaving here in 1978 to serve as a political assistant to former president Gerald R. Ford in Palm Springs, Calif., during the congressional campaigns.

She returned to Washington after the 1978 elections, but was unable to resume work because of ill health. She went back to her native California in later 1979.

Miss Lopat was born in Burbank and grew up in Upland, Calif. She graduated from Colorado College in Colorado Springs, where she was elected to Phi Beta Kappa.

She is survived by her parents, Dr. and Mrs. Stanley P. Lopat, and a sister, Renee R. all of Upland; two other sisters, Camille E. Melkonian of South Laguna, and Constance A., of Laguna Beach, and her grandparents, Mary A. Lopat, of Claremont, Calif., and Mr. and Mrs. G. W. Epeneter, of Laguna Beach.

The family suggests that expressions of sympathy be in the form of contributions to the Marianne L. Lopat Memorial Fund at Colorado College, Colorado Spring.