In 1596, he accompanied his cousin, Henry Clinton, 2nd Earl of Lincoln, on a diplomatic mission to the Landgrave of Hesse.
He travelled with Edward Seymour, 1st Earl of Hertford to Brussels in 1605 on his embassy to Albert VII, Archduke of Austria.
Upon the accession of James I, Lord Burghley petitioned the king to recognise Fiennes as the legitimate successor to his ancestor's barony.
In 1603, James I issued letters patent confirming Fiennes and the heirs of his body as Baron Saye and Sele.
Additionally, the 1603 patent effectively created a new title for Fiennes, and barred him from claiming the precedence and privilege of the 1447 creation.