Ralph de Monthermer and his marriage to Joan of Acre (daughter of Edward Longshanks and Eleanor of Castille) is the closest to what your asking. She had been married before to Gilbert de Claire, one of the wealthiest and most influential nobles of the time, but was left a widow. She met and fell in love with a squire in service of her late husband, Ralph. Ralph and Joan married in secret but her father was already attempting to arrange another marriage for her. When he discovered affair, he imprisoned Ralph but eventually relented after realizing she was already pregnant. Ralph and Joan would have 4 kids and he would gain titles but Joan would later die in childbirth. Their children were considered nobles as well.

edited for clarification

Catherine of Valois was a French princess and widow of King Henry V of England. After his death she married, or at least had several children with, Owen Tudor, who was from Welsh nobility but may have been working in her household at the time. Their descendants eventually became the Tudor dynasty.