This was an Irish peerage and was not to entitle him to an automatic seat in the House of Lords on his 21st birthday in 1805.
In 1806 he was one of the contenders for William Pitt the Younger's parliamentary seat at Cambridge University but made way for Lord Palmerston.
At the general election of that year he was instead returned for Ripon, a seat controlled by the Allanson family.
However, FitzWilliam and his supporters managed to obtain evidence of corruption and had Headley unseated on petition in 1808.
He died in April 1840, aged 55, and was succeeded in the titles by his nephew, Mark.