Explanation
No. King John agreed to Magna Carta in 1215, but he did not authenticate it with a handwritten signature. The document was authenticated with his royal seal.
UK Parliament's account of the surviving charters explicitly states that neither John nor the witnesses signed them. A special official applied the seal. The US National Archives independently describes the 1215 agreement as sealed.
Calling this the 'signing of Magna Carta' can obscure how the document was made official. Scribes wrote the text on parchment; the royal seal authenticated it. The distinction concerns the method of authorization, not whether John agreed to the charter.
Key takeaway
Magna Carta received royal authentication through a seal. A modern image of King John signing it with a pen misrepresents that process.