In 1918 Sinclair married Marigold (1897–1975), daughter of Lieutenant-Colonel James Stewart Forbes and Lady Angela Forbes.
They had four children: the Hon.
Catherine (1919–2007), the Hon.
Elizabeth (1921–1994), Robin (1922–1995), and the Hon.
Angus (born 1925).
Sinclair was one of the largest landowners in the United Kingdom, owning an estate of about 100,000 ac (40,000 ha) in Caithness.
He was handsome and charming and regarded as a daredevil, but in private life, he was rather shy, reserved and antisocial, with a slight speech impediment.

In the 1990s, his grandson, John Sinclair, entered politics and sat from 2001 to 2015 as the Liberal Democrat MP for his grandfather's seat, Caithness, Sutherland and Easter Ross.
Sinclair's granddaughter, Veronica Linklater, Baroness Linklater of Butterstone (daughter of the Hon.
Elizabeth Sinclair), has also become a Liberal Democrat politician.
In 1918 Sinclair married Marigold (1897–1975), daughter of Lieutenant-Colonel James Stewart Forbes and Lady Angela Forbes. They had four children: the Hon. Catherine (1919–2007), the Hon. Elizabeth (1921–1994), Robin (1922–1995), and the Hon. Angus (born 1925). Sinclair was one of the largest landowners in the United Kingdom, owning an estate of about 100,000 ac (40,000 ha) in Caithness. He was handsome and charming and regarded as a daredevil, but in private life, he was rather shy, reserved and antisocial, with a slight speech impediment.
In the 1990s, his grandson, John Sinclair, entered politics and sat from 2001 to 2015 as the Liberal Democrat MP for his grandfather's seat, Caithness, Sutherland and Easter Ross. Sinclair's granddaughter, Veronica Linklater, Baroness Linklater of Butterstone (daughter of the Hon. Elizabeth Sinclair), has also become a Liberal Democrat politician.
Archibald Sinclair, 1st Viscount Thurso