Born on 15 January 1954 at Hammersmith in London, Jackman lived most of his first eight years in Whitton in the county of Middlesex. Registered as an Australian at birth, he lived in Melbourne, Australia, from age 8 to 13 and subsequently in Pennsylvania, USA. Up to age 13 he traveled often and far, passing three times through the Suez Canal, with extended sojourns in Iowa, South Australia, and Tennessee. He has worked and published in Germany as well as in the United States.

He is known for contributions to medieval European legal history, in particular the law of inheritance in public office. Involvement in the controversies surrounding heritability has led to significant contributions in historical linguistics, historical method, and philosophy of history. In addition to addresses at history conferences on two continents, Jackman has read a paper at the annual convention of the American Association of Applied Linguistics. He has composed for classical guitar, with pieces premiering in the 1970s.