[2] Sidney served in the army in the Netherlands during his father's governorship of Flushing, and was given command of an English regiment in the Dutch service in 1616.

In 1620 he had a disagreement with James Hay, Viscount Doncaster, who was his brother-in-law, having married Lucy Percy.

They argued at Petworth, Sidney struggled with Hay's servants and left his hat behind.

[2] In 1615, Sidney married Lady Dorothy Percy, the daughter of Henry Percy, 9th Earl of Northumberland and Dorothy Devereux, who was a daughter of Robert Devereux, 2nd Earl of Essex.

[4] They had twelve children, including: Philip and Algernon supported the Parliamentary cause in the English Civil War.