Monsell was born at Cahirciveen, County Kerry, to a wealthy Anglo-Irish family.
His father, William Thomas Monsell (1843–1887), was a magistrate and inspector of facturers; his wife, Elinor Vere, was daughter of Hon.
John Samuel Bewley Monsell (1811–1875), vicar of Egham, Surrey, was first cousin to William Monsell, 1st Baron Emly; William Thomas served as Lord Emly's private secretary during his time as Postmaster General.
[citation needed] He served as a lieutenant in the 12th Battalion (The Rangers), London Regiment of the British Army during the First World War, having trained with the Inns of Court Officers' Training Corps;[6] he married the novelist Margaret Irwin in 1927.
[citation needed] Monsell's edition of The Rose and the Ring by William Makepeace Thackeray was largely successful.