By the time he was twenty-four he had served six monarchs, including the King of Denmark and Gustavus Adolphus of Sweden.
He returned to Scotland disillusioned by lack of pay, and in 1630 obtained a position as a chorister of the Chapel Royal at Stirling, from Charles I.
[1] David Stevenson, Mercer's biographer in the Oxford Dictionary of National Biography, says that "This musical side to his interests makes it likely that he was the 'William Merser, musician' who was admitted a burgess of Edinburgh in 1631 (Wood, 3.95).
"[1] He fought for the Protestant cause in the Irish Rebellion of 1641, a conflict in which his brother Robert, a Presbyterian minister, and his family were killed,[2] but left Ireland to join the Parliamentary side in the First English Civil War.
But it was not completed by his death and David Stevenson says "That this never appeared need be little lamented, for 'Mercer's writings are mainly valuable for their autobiographical details.