In 1961 Thomas married Nan Kerr, with whom he had three sons and one daughter. She died in 2000. In 2005, he married fellow Liberal Democrat peer Baroness Walmsley.
Lord Thomas was a vice-president of the Lloyd George Society until February 2012, when he was elected president.
Lord Thomas is the honorary chairman of the Association of Military Court Advocates.
Lord Thomas is the honorary president of the Hong Kong Welsh Male Voice Choir[4]