Acton was born to a prominent Anglo-Italian-American family of baronets, later raised to the peerage as Barons Acton of Aldenham at Villa La Pietra, his parents' house one mile outside the walls of Florence, Italy.
He claimed that his great-great-grandfather was Commodore Sir John Acton, 6th Baronet (1736–1811), who married his niece, Mary Anne Acton, and who was prime minister of Naples under Ferdinand IV and grandfather of the Roman Catholic historian Lord Acton.
This relationship has been disproven; Harold Acton in fact descends from Sir John Acton's brother, General Joseph Edward Acton (1737–1830).
Both of these brothers served in Italy, and are from the Shropshire family of Actons.

His father was the successful art collector and dealer Arthur Acton (1873–1953), the illegitimate son of Eugene Arthur Roger Acton (1836–1895), counsellor to the Egyptian Ministry of Agriculture and Commerce.
His mother, Hortense Lenore Mitchell (1871–1962), was the heiress of John J. Mitchell, a president of the Illinois Trust and Savings Bank, an appointed member of the Federal Advisory Council,, and a trustee of the Art Institute of Chicago (1908–1909).
Arthur Acton met Hortense in Chicago while helping to design the Italianate features of the bank's new building in 1896, and the Mitchell fortune allowed Arthur Acton to buy the remarkable Villa La Pietra on the hills of Florence, where Harold Acton lived for much of his life.
The only modern furniture in the villa was in the nurseries, and that was disposed of when the children got older (Harold's younger brother William Acton was born in 1906).
Acton was born to a prominent Anglo-Italian-American family of baronets, later raised to the peerage as Barons Acton of Aldenham at Villa La Pietra, his parents' house one mile outside the walls of Florence, Italy. He claimed that his great-great-grandfather was Commodore Sir John Acton, 6th Baronet (1736–1811), who married his niece, Mary Anne Acton, and who was prime minister of Naples under Ferdinand IV and grandfather of the Roman Catholic historian Lord Acton. This relationship has been disproven; Harold Acton in fact descends from Sir John Acton's brother, General Joseph Edward Acton (1737–1830). Both of these brothers served in Italy, and are from the Shropshire family of Actons.
His father was the successful art collector and dealer Arthur Acton (1873–1953), the illegitimate son of Eugene Arthur Roger Acton (1836–1895), counsellor to the Egyptian Ministry of Agriculture and Commerce. His mother, Hortense Lenore Mitchell (1871–1962), was the heiress of John J. Mitchell, a president of the Illinois Trust and Savings Bank, an appointed member of the Federal Advisory Council,, and a trustee of the Art Institute of Chicago (1908–1909). Arthur Acton met Hortense in Chicago while helping to design the Italianate features of the bank's new building in 1896, and the Mitchell fortune allowed Arthur Acton to buy the remarkable Villa La Pietra on the hills of Florence, where Harold Acton lived for much of his life. The only modern furniture in the villa was in the nurseries, and that was disposed of when the children got older (Harold's younger brother William Acton was born in 1906).
Harold Acton