Former attorney general John N. Mitchell, now a consultant with a Washington research firm, was sued yesterday for alleged breach of contract by book publisher Simon and Schuster after Mitchell allegedly failed to deliver his Watergate memoirs for which he had been paid $50,000 in advance.

According to the lawsuit, filed in New York State Supreme Court, Mitchell signed an agreement with the New York publisher to deliver two copies of a 100,000-word manuscript -- "Untitled Memoirs of the Nixon Years" -- by April 1, 1976. The suit says Mitchell received an advance payment of $50,000 at the time of the signing (July 21, 1975) and was to receive an additional $100,000 as the work progressed.

When the manuscript was not delivered, the lawsuit alleges, Simon and Schuster sent a written notice to Mitchell's agent, International Creative Management, terminating the agreement and demanding that Mitchell repay the $50,000 advance.

"Despite repeated requests . . . the defendent has refused to repay," the lawsuit claims.

Mitchell, reached at his Washington office yesterday, declined to comment on the litigation. Last March, he and a partner, James M. Tully, started the firm Global Research International Inc., which subleases office space at 2550 M St. NW.

Simon and Schuster said yesterday that Mitchell's editor was to be best-selling author Michael Korda ("Success!"). Korda could not be reached for comment yesterday.

Mitchell, 67, was originally sentenced to 2 1/2 to 8 years for conspiracy, obstruction of justice and lying under oath for his part in the Watergate affair. Judge John Sirica later reduced his sentence to one to four years.

Mitchell served 19 months at the minimum security institution at Maxwell Air Force Base in Alabama. He was freed on parole Jan. 20, l979, telling reporters, "From henceforth, don't call me. I'll call you."