Ronald Charles Grant, 10th Baron de Longueuil was born on 13 March 1888 at Pau, France. He was the son of John Moore Charles de Bienville Grant. He married Ernestine Hester Maud Bowes-Lyon, daughter of the Hon. Ernest Bowes-Lyon and Isobel Hester Drummond, on 4 October 1918. He was educated at Elizabeth College, Guernsey. He was in the French Foreign Legion and was a civil engineer.
He succeeded to the title of Baron de Longueuil on 17 October 1935. He fought in the Second World War as an officer in the service of the 3rd Battalion, King's Own Yorkshire Light Infantry.
Ancestry
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| | | | | | | | | 16. David Alexander Grant |
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| 8. Charles William Grant, 5th Baron de Longueuil | |
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| 17. Marie-Charles Le Moyne, 4th Baroness de Longueuil |
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| 4. Charles James Irwin Grant, 6th Baron de Longueuil | |
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| 18. John Coffin |
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| 9. Caroline Coffin | |
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| 19. Anne Matthews |
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| 2. John Charles Moore Grant, 9th Baron de Longueuil | |
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| 20. Peter Trapman |
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| 10. Louis Trapman | |
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| 21. Eva Maria Lemme |
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| 22. John Elias Moore |
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| 11. Mary Bowen Moore | |
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| 23. Frances Bowen |
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| 1. Ronald Charles Grant, 10th Baron de Longueuil | |
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| 24. Eustace Barron |
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| 12. William Leonard Barron | |
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| 25. Catalina Cantillon |
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| 6. Joseph Barron | |
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| 13. Mary Catherine Murphy | |
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| 3. Maria-Carlotta Barron | |
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| 14. William Robert Walkinshaw | |
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| 7. Roberta Walkinshaw | |
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| 15. Francisca Garriz | |
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| 8. Charles William Grant, 5th Baron de Longueuil | |
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| 17. Marie-Charles Le Moyne, 4th Baroness de Longueuil |
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| 4. Charles James Irwin Grant, 6th Baron de Longueuil | |
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| 18. John Coffin |
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| 9. Caroline Coffin | |
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| 19. Anne Matthews |
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| 2. John Charles Moore Grant, 9th Baron de Longueuil | |
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| 20. Peter Trapman |
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| 10. Louis Trapman | |
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| 21. Eva Maria Lemme |
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| 5. Anne Marie Catherine Trapman | |
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| 22. John Elias Moore |
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| 11. Mary Bowen Moore | |
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| 23. Frances Bowen |
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| 1. Ronald Charles Grant, 10th Baron de Longueuil | |
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| 24. Eustace Barron |
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| 12. William Leonard Barron | |
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| 25. Catalina Cantillon |
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| 6. Joseph Barron | |
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| 13. Mary Catherine Murphy | |
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| 3. Maria-Carlotta Barron | |
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| 14. William Robert Walkinshaw | |
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| 7. Roberta Walkinshaw | |
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| 15. Francisca Garriz | |
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