He was active in the Society for the Propagation of Christian Knowledge and greatly concerned with the lasciviousness and "impiety" of the English stage.
He wrote, anonymously, to rail against stage performances, including correspondence with Daniel Defoe and the then Archbishop of Canterbury, Thomas Tenison.
Like William Law's later A Serious Call to a Devout and Holy Life (1728), Melmoth's book emphasized personal renewal and religious awakening.
In 1719, he was made a Bencher of Lincoln's Inn and was soon recognised as one of the leading chancery counsel of the day.
He devoted considerable energies to the physical and financial improvement of the Inn and continued to work up until a few days before his death on 6 April 1743.