Mary Foulis was the only child and heiress of Sir William Foulis, 8th Bt., of Ingleby Manor. Co. York, by his wife Mary Jane Ross, 2nd daughter of Gen Sir Charles Lockhart-Ross of Balnagowan, 7th Bt. of Carstairs.
She married Philip Sidney, 2nd Baron De L'Isle and Dudley of Penshurst, son of Philip Charles Sidney, 1st Baron De L'Isle and Dudley of Penshurst and Lady Sophia Fitz-Clarence, on 23 April 1850.
Under the terms of her father’s will, her husband assumed the additional name of Foulis after that of Sidney by Royal Warrant, 6 Jun 1850 (i.e. Sidney-Foulis), but relinquished it after her death.
Upon her husband's succession to the title of 2nd Baron De L'Isle and Dudley of Penshurst, she was styled Lady De L'Isle and Dudley.
The Right Honourable Mary Foulis, Baroness De L’Isle and Dudley (wife of the Right Honourable Philip Sidney Foulis, Baron De L’Isle and Dudley), of Ingleby Manor, Yorkshire, died intestate at 8 Lennox Gardens, Middlesex (now Knightsbridge area of London). Probate was granted to her husband. Effects £2747 3s. 2d.

Mary Foulis was the only child and heiress of Sir William Foulis, 8th Bt., of Ingleby Manor. Co. York, by his wife Mary Jane Ross, 2nd daughter of Gen Sir Charles Lockhart-Ross of Balnagowan, 7th Bt. of Carstairs.
She married Philip Sidney, 2nd Baron De L'Isle and Dudley of Penshurst, son of Philip Charles Sidney, 1st Baron De L'Isle and Dudley of Penshurst and Lady Sophia Fitz-Clarence, on 23 April 1850.
Under the terms of her father’s will, her husband assumed the additional name of Foulis after that of Sidney by Royal Warrant, 6 Jun 1850 (i.e. Sidney-Foulis), but relinquished it after her death.
Upon her husband's succession to the title of 2nd Baron De L'Isle and Dudley of Penshurst, she was styled Lady De L'Isle and Dudley.
The Right Honourable Mary Foulis, Baroness De L’Isle and Dudley (wife of the Right Honourable Philip Sidney Foulis, Baron De L’Isle and Dudley), of Ingleby Manor, Yorkshire, died intestate at 8 Lennox Gardens, Middlesex (now Knightsbridge area of London). Probate was granted to her husband. Effects £2747 3s. 2d.