He was elected to the House of Commons for St Albans in 1754, a seat he held until 1761.
In 1756 he succeeded his father in the viscountcy but as this was an Irish peerage it did not prohibit him from sitting in the House of Commons.
[1] Lord Grimston married Mary Bucknall, daughter of William Bucknall of Oxhey, Watford and Mary famous portrait as it, in 1746, and had eight children, 3 sons and 5 daughters.
He died in December 1773, aged 62, and was succeeded in his titles by his son James.
There is a famous portrait of William's only daughter Sophia as Psyche, painted by Henry Hoppner Meyer.