GROTON -- Lillian A. Mitchell, 90, a longtime Groton resident, died yesterday, May 10, at Apple Valley Nursing and Rehab Center. She was the wife of William A. "Buff" Mitchell, who died in 1980.She was born in the western section of Groton on Aug. 7, 1914, daughter of the late Edmund and Mary (Connolly) DeRoche. She graduated from Groton High School and attended Katherine Gibbs School in Boston.For 43 years, Mrs. Mitchell was employed with the former Groton Leather Board.She was a parishioner of St. James parish in Groton. She enjoyed traveling, playing bridge and some of the programs at the Groton Senior Center.Her survivors include a niece, Margaret A. Barrett of Groton; three nephews, Thomas Mitchell of Springfield, Joseph Mitchell of Merrimack, N.H., and William "Bud" Cornellier of Ayer; two grandnieces, Kathleen Barrett of Leominster and Suzanne Joyner of Lancaster; and two great-grandnephews, William and Thomas Joyner.She was also the aunt of the late Matthew Barrett.