On 14 August 1762, he married Lady Charlotte Greville (d. 1763), the daughter of Francis Greville, 1st Earl of Warwick (1719–1773).
They had two sons, both of whom died in infancy.

After his first wife's early death, he remarried to Anne Dashwood (1743–1830), daughter of Sir James Dashwood, 2nd Baronet, on 13 June 1764.
Together, John and Anne were the parents of sixteen children:
Lady Catherine Stewart (1765–1836), who married Sir James Graham, 1st Baronet in 1781.

Hon.
Alexander Stewart (1766–1766), who died in infancy.

Lady Susan Stewart (1767–1841), who married George Spencer-Churchill, 5th Duke of Marlborough in 1791.

Adm. George Stewart, 8th Earl of Galloway (1768–1834), a naval commander and politician.

Lady Anne Harriet Stewart (1769–1850), who married Lord Spencer Chichester in 1795
Lady Elizabeth Euphemia Stewart (1771–1855), who married William Philips Inge in 1798.

Hon.
Leveson Keith Stewart (1772–1780), who died young.

Lady Georgiana Frances Stewart (1776–1804).

Lt.-Gen.
Hon.
Sir William Stewart (1774–1827), a military officer who was the first Commanding Officer of the Rifle Corps, a Division Commander in the Peninsular War, and an MP.

Rt.
Rev. Hon.
Charles James Stewart (1775–1837), who became the Bishop of Quebec.

Lady Charlotte Stewart (1777–1842), who married Sir Edward Crofton, 3rd Baronet in 1801.

Lady Caroline Stewart (1778–1818), who married Rev. Hon.
George Rushout-Bowles in 1803; mother of George Rushout, 3rd Baron Northwick.

Hon.
Montgomery Granville John Stewart (1780–1860).

Hon.
Edward Richard Stewart (1782–1851), an MP who became a Commissioner of the Victualling Board and Paymaster and Inspector-General of the Marines.

Lt.-Col.
James Henry Keith Stewart (1783–1836), who became a Tory Member of Parliament.

Lady Georgiana Charlotte Sophia Stewart (1785–1809), who married Col.
Hon.
William Bligh (1775-1845), son of John Bligh, 3rd Earl of Darnley, in 1806.

Lord Galloway died on 13 November 1806 and was succeeded in his titles by his eldest surviving son, the Admiral George Stewart, 8th Earl of Galloway, who married Lady Jane Paget, the daughter of Henry Paget, 1st Earl of Uxbridge, and sister of Henry Paget, 1st Marquess of Anglesey.
On 14 August 1762, he married Lady Charlotte Greville (d. 1763), the daughter of Francis Greville, 1st Earl of Warwick (1719–1773). They had two sons, both of whom died in infancy.
After his first wife's early death, he remarried to Anne Dashwood (1743–1830), daughter of Sir James Dashwood, 2nd Baronet, on 13 June 1764. Together, John and Anne were the parents of sixteen children:
Lady Catherine Stewart (1765–1836), who married Sir James Graham, 1st Baronet in 1781.
Hon. Alexander Stewart (1766–1766), who died in infancy.
Lady Susan Stewart (1767–1841), who married George Spencer-Churchill, 5th Duke of Marlborough in 1791.
Adm. George Stewart, 8th Earl of Galloway (1768–1834), a naval commander and politician.
Lady Anne Harriet Stewart (1769–1850), who married Lord Spencer Chichester in 1795
Lady Elizabeth Euphemia Stewart (1771–1855), who married William Philips Inge in 1798.
Hon. Leveson Keith Stewart (1772–1780), who died young.
Lady Georgiana Frances Stewart (1776–1804).
Lt.-Gen. Hon. Sir William Stewart (1774–1827), a military officer who was the first Commanding Officer of the Rifle Corps, a Division Commander in the Peninsular War, and an MP.
Rt. Rev. Hon. Charles James Stewart (1775–1837), who became the Bishop of Quebec.
Lady Charlotte Stewart (1777–1842), who married Sir Edward Crofton, 3rd Baronet in 1801.
Lady Caroline Stewart (1778–1818), who married Rev. Hon. George Rushout-Bowles in 1803; mother of George Rushout, 3rd Baron Northwick.
Hon. Montgomery Granville John Stewart (1780–1860).
Hon. Edward Richard Stewart (1782–1851), an MP who became a Commissioner of the Victualling Board and Paymaster and Inspector-General of the Marines.
Lt.-Col. James Henry Keith Stewart (1783–1836), who became a Tory Member of Parliament.
Lady Georgiana Charlotte Sophia Stewart (1785–1809), who married Col. Hon. William Bligh (1775-1845), son of John Bligh, 3rd Earl of Darnley, in 1806.
Lord Galloway died on 13 November 1806 and was succeeded in his titles by his eldest surviving son, the Admiral George Stewart, 8th Earl of Galloway, who married Lady Jane Paget, the daughter of Henry Paget, 1st Earl of Uxbridge, and sister of Henry Paget, 1st Marquess of Anglesey.
John Stewart, 7th Earl of Galloway