Portman was the son of Edward Portman, of Bryanston and Orchard in Dorset, and his wife Lucy, daughter of Reverend Thomas Whitby.
He was the son of Viscount Portman and Lady Emma Lascelles and was educated at Durham University in England.
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Portman was born at Bryanston, Dorset, the son of Edward Portman, 1st Viscount Portman and his wife Lady Emma Lascelles.