She was the mother of three of his children, including his heir, Archibald Campbell, 1st Marquess of Argyll, the de facto head of the government in Scotland throughout most of the conflict known as the Wars of the Three Kingdoms.

Lady Agnes and her sisters were so famed for their beauty throughout Scotland that they were known as the "pearls of Lochleven".

The title had been forfeited in 1581 when James Douglas, 4th Earl of Morton, regent of Scotland, was executed and attainted for his part in the murder of Henry Stewart, Lord Darnley, in 1567.

[5] Despite Agnes's religion, he commanded the royal troops which fought against the Catholic rebels led by George Gordon, 1st Marquess of Huntly, in the Battle of Glenlivet on 3 October 1594.

In 1599, when she was twenty-five years old, Agnes's portrait was painted by Flemish artist Adrian Vanson.