Corrected 26 September 2026: Mary was executed on 8 February 1587, not in 1586. Speculative consequences are identified below as hypotheses.
The historical starting point
Mary, Queen of Scots, was executed at Fotheringhay on 8 February 1587. Her captivity in England and implication in plots against Elizabeth I, including the Babington Plot, form the context for that decision. See English Heritage's account of Mary's captivity and the Royal Family's historical biography.
An alternate history needs to say precisely what changes. Here the proposed change is that Mary remains alive after the date of her actual execution. It does not assume that she is released, gains a throne or commands a successful rebellion.
Three possibilities to test
Continued imprisonment. Mary survives but remains under guard. This requires the fewest additional changes to the immediate situation. It still leaves questions about security, correspondence and political pressure.
A negotiated arrangement. Her survival becomes part of a diplomatic proposal. This is a more demanding hypothesis: who would guarantee the terms, what would they gain, and why would Elizabeth's government trust them?
Renewed attempts to use her claim. Supporters might continue to see political value in her survival. That possibility does not establish that a plot would succeed or that civil war would follow.
These are analytical options, not predictions. Each needs evidence about the actors' resources and interests before it can become a plausible narrative.
Keep the timescale manageable
Start with the following months, rather than jumping straight to an altered British Empire or different world wars. Every additional decade introduces decisions that cannot be inferred from one changed execution.
A useful exercise is to draw two columns: consequences directly enabled by survival, and consequences requiring another assumption. Put a successful seizure of power in the second column, not the first.
Our Black Death counterfactual guide uses the same method: establish the actual record, change one condition, and make uncertainty visible. Historical fiction can explore those possibilities without presenting them as inevitable history.