Mark David O'Neil, 62, of Bolton, passed away unexpectedly from natural causes at his home in Bolton on October 28, 2020. Mark was born and raised in Concord, Massachusetts, the son of the late S. Elizabeth (Garrity) O'Neil and the late Gerald P. O'Neil. He is survived by his beloved best friend Greater Swiss Mountain Dog, Barrett; his former wife of 20 years and close friend, Sarah (Sally) Jeffrey O'Neil; stepson Christopher K. Durlacher, wife Nora Elton, daughter, Zoe, and son, Jasper, of Milton, Massachusetts; stepdaughter Katherine Durlacher McCann, husband Michael McCann, son, Henry and daughter, Quin of Watertown, Massachusetts; and stepdaughter Margaret Durlacher Rixey, and husband Eppa Rixey V of Cambridge, Massachusetts. He was the much loved brother of Nancy (O'Neil) Baker of Concord, Massachusetts and Stephen P. O'Neil, and wife Nancy, of Westford, Massachusetts; and uncle to Amy Rumpf, and husband David, of Macungie, Pennsylvania; Amanda Fontana, and husband Mike, of Bridgewater, Massachusetts; Kyle O'Neil of Denver, Colorado; and Lance Gamester, and wife Heather, of Wilmington, North Carolina as well as several great-nieces. Mark graduated from Concord-Carlisle High School in 1976 and went on to join the U.S. Army, where he completed Airborne Training, served in Panama and other countries, and was a personal driver to a Colonel. After serving his country, Mark made his home in Concord and Sudbury. He built an accomplished career as a systems analyst for Microsoft Corporation, Teradyne Inc., and Hologic Inc., traveling across the U.S., China and Thailand. He was a proud stepdad to his and Sally's three children, spouses, and four grandchildren, to whom he was lovingly known as "Grey Dog'. He and Sally raised Barrett, their Greater Swiss Mountain Dog, showing him around New England until he received his AKC Grand Championship. Favorite pastimes were riding Ducati motorcycles, and countless hours of hiking in the woods near Fairhaven Bay in Concord, Massachusetts with his dalmatian, Porter, and later, with Barrett. In the recent past, he and Barrett made a home in Bolton, Massachusetts on a beautiful property surrounded by woodlands and animals. Mark was proud of his decades of hard work in recovery – he was always very involved and helped many people who shared that journey with him. Services will be determined at a later date. In lieu of flowers, donations in Mark's memory may be made to Greater Swiss Mountain Dog Rescue Foundation , 19010 70th Ave East, Bradenton, FL 34211. Arrangements are under the care of Dee Funeral Home & Cremation Service of Concord.