In a letter to Mary of Guise written in 1553, Sinclair mentions her long service starting as nurse to her short-lived son Prince James, born in 1540.
At Linlithgow on 22 March 1543, Mary of Guise asked Jean Sinclair to unwrap the queen from her swaddling clothes to show the English ambassador Ralph Sadler that she was a healthy infant.
As the war with England now known as Rough Wooing continued, Mary and her household including her governess Janet Stewart, Lady Fleming went to Dumbarton Castle on the Clyde and sailed to France.
[5] When Jean Sinclair wrote to Mary of Guise from the Château de Blois in 1553 she signed her letter "Jaine Syncler, nureis to our soveraine lady".
[8] The young queen's household in France was re-organised again in 1554 by Comptroller Astier, who subsequently came to Scotland as director of military finance during the refortification of Eyemouth and Inchkeith.