James VI and I, of Scotland and England respectively, was Mary Stuart's son. Given that Mary was executed for treason and an attainder (which resulted in corruption of blood) was automatic in cases of treason, how was Mary's son able to succeed in her titles and eventually inherit not just one but both thrones?
Part One: Chronology.
King James VI & I did become king of England and Ireland on 24 March 1603 about 16 years after Mary Queen of Scots was beheaded on 8 February 1587.
But James became King of Scotland on 24 July 1567, about 20 years before his mother Mary Queen of Scots was beheaded 8 February 1587. The enemies of Mary Queen of Scots revolted against her, defeated her, and forced her to abdicate, and then made her one-year-old son James the King of Scotland, ruling for him until he came of age.
So since James became King of Scotland almost 20 years before any hypothetical act of attainder against his mother Mary Queen of Scotland, such a hypothetical act of attainder wouldn't prevent James from becoming King of Scotland in 1567.
Part Two: England and Scotland Were Separate & Independent Countries.
I also note that Mary was beheaded for treason against Queen Elizabeth I of England and Ireland. Thus any hypothetical act of attainder against Mary for treason against the Queen of England would not make her guilty of treason against the monarch of Scotland, whether that she herself was considered to be still the rightful monarch of Scotland or James VI was considered to be the rightful monarch of Scotland.
Neither Mary herself or James VI was disbarred from the throne of Scotland by any real or unreal treason she might have committed against England.
As far as I know Mary was never accused of acts of treason against herself while she was Queen of Scotland. And possibly the supporters of those who deposed her in favor of her son James could consider her attempts to regain her throne treason against the king who replaced. But obviously they would not consider such efforts to regain the throne reason to disbar her heir from the crown, because they had put her heir on the throne in place of her, so it would be illogical to assume that James inherited any sort of legal guilt as a result of treason committed by another against him.
So this establishes that any hypothetical act of attainder against Mary for Treason against Elizabeth of England would have had absolutely no effect in removing James from the throne of another kingdom, Scotland, where he had been the monarch for almost 20 years already.
Part Three: No Bill of Attainder.
The answer by Mark Olson shows that no act of attainder was passed against Mary, and thus that a nonexistent act of attainder could have no effect in keeping James from the Throne of England.
An act of attainder was a special law decreeing that someone was guilty of treason "because we say so" and confiscating all their property and "corrupting their blood", making their heirs unable to inherit their property.
Considering the present state of politics in the USA, it would be terrible if the majority in Congress at a moment could pass bills of attainder against members of the other party. That is why the US Constitution forbids bills of attainder.
As Mark Olson says, Mary was convicted by a jury of English nobles and so no bill of attainder was passed against her, there was no corruption of blood, and James's hereditary right to the throne of England was not affected.
Part Four: What One Parliament Has Done, Another Parliament Can Undo.
If I remember correctly, bills of attainder were often eventually reversed, so that heirs of the attained could inherit titles and property.
So if the English parliament had passed a bill of attainder against Mary Queen of Scots, a later English parliament which desired James to become King of England & Ireland could have passed a bill reversing the corrupution of blood in the earlier bill of attainder.
King James was not disbarred from becoming king of either Scotland or England by the sentence against Mary Queen of Scots.
- Mary was not the subject of a Bill of Attainder. She was convicted of treason by a jury of English noblemen.
- The rules about succession rules were still vague in 1600. Succession was set by the monarch and rubber-stamped (usually) by Parliament. No one disputed that Queen-in-Parliament could overturn any taint of that sort that existed. What rules there were favored legitimate descent with a very strong bias towards Protestants.
- In any event, Elizabeth never named a successor or succession rules for Parliament to approve. James was selected more-or-less by unanimous consent of the political elite surrounding Elizabeth. Once they had become convinced that Elizabeth was not going to name James -- or anyone else -- as her successor, many of her advisors began corresponding with James in secret when it became obvious that her death was near. (No one wanted a return to Wars of the Roses days!) Basically, everyone who mattered -- i.e., could muster enough force to interfere -- said, "Yup. Hooraw for King James!"
The bottom line is that James was the absolutely obvious choice by descent, he had already shown himself to be a competent king in Scotland, he was Protestant, most of the political elite favored him, and there was no legal impediment. I suppose, technically, this was a coup, but there was no one around to object to it.