William Rulon Williamson, 91, who helped set up the Social Security Board under President Franklin D. Roosevelt, died July 26 at a convalescent home in Windsor, Conn. He had arteriosclerosis.

A former resident of Suitland, he had lived in Windsor since 1974.

Mr. Williamson was an assistant actuary for The Travelers Insurance Cos., in Hartford, Conn., when he was loaned to the federal government's Committee on Economic Security. He helped work out the unemployment insurance program.

In 1935, he was appointed chief actuary of the system, now known as the Social Security Administration. He resigned in 1947 and became president of the Wyatt Co. of Consulting Actuaries in Washington.

He left the Wyatt Co. in 1950 and worked as a private consultant.

Mr. Williamson wrote numerous articles on insurance and business. He belonged to the American Statistical Association, the Bureau of Economic Research, the Tax Institute, the Population Association of America and the Academy of Political Scientists. He had been a Guggenheim fellow. He also had been a fellow of the Society of Acturaries and a member of the Cosmos Club.

He was born in Wales, N.Y. He earned bachelor's and master's degrees from Wesleyan University in Middletown, Conn.

He is survived by two sons, William R. Jr., of Pelham Manor, N.Y., and Addison H., of Beaumont, Tex., and six grandchildren.

The family suggests that expressions of sympathy be in the form of contributions to the Potomac Appalachian Trail Club, in which he had been active for 35 years.