SANTA BARBARA, Calif. (AP) _ John J. ''Jack'' Mitchell, a founder of the forerunner of United Airlines as well as of an exclusive riding club that includes President Reagan, died at his home over the weekend. He was 87.

Mitchell, who died Sunday, helped establish National Air Transport, which later became United Airlines. He became a member of the board of directors of United Airlines in 1937 and remained a director until 1979.

In 1930, he co-founded Rancheros Visitadores, an organization of horse riders whose membership has included presidents and national business leaders as well as local riding enthusiasts. Reagan has been an honorary member since 1980, Rancheros assistant secretary Horace Rupp said Monday.

Mitchell had ridden with the Rancheros until 1961.

Born April 28, 1897, in Chicago, Mitchell attended Yale University, then served as a Navy aviator in World War I. He was a third-generation banker who worked at the Illinois Merchant Trust and Savings Bank, which became Continental Illinois Bank and Trust Co.

A philantrophist, Mitchell once donated a Spanish coffer that had been owned by the family of Father Junipero Serra, founder of California missions, to the Santa Barbara Historical Society.

He married Lolita Armour in 1921 and later divorced. In 1942, he married Olga Voronseva Varchavshia and settled in Santa Barbara.

Survivors include his daughter, Lolita ''Tita'' M. Lanning of Santa Barbara; sons John J. Mitchell Jr. of Santa Barbara and James J. Mitchell of Mountain View; brother William H. Mitchell and seven grandchildren.

A memorial service will be at 2 p.m. Thursday at All Saints by the Sea Episcopal Church in Montecito, church office manager Pat Cory said Monday.