Keith gained a degree in Archaeology from Southampton University in 1973 and upon graduation became a Field Officer for the newly formed Norfolk Archaeological Unit, moving to the Suffolk Archaeological Unit in 1974 to take up the post of Urban Archaeologist. Between 1974 and 1990 he directed 35 excavations in Saxon and medieval Ipswich. In 1991 he became County Archaeologist at Suffolk County Council, a post he held until retirement in 2012.
Keith has served as a Trustee with many local organisations including the West Stow Anglo-Saxon Village Trust and the Sutton Hoo Research Trust. He founded Ipswich Archaeological Trust in 1982 and remains a Trustee and Honorary Secretary. He was elected as a Fellow of the Society of Antiquaries in 1990. He joined the Board of Trustees in 2019.