George Pretyman was born in 1750 in Bury St. Edmunds, Suffolk, the eldest son of George Pretyman and Susan Hubbard. Pretyman was educated at Bury St. Edmunds Grammar School and matriculated at Pembroke College in 1768. He took the Mathematics tripos in 1772 and was Senior Wrangler, also receiving the 1st Smith's Prize. He was awarded his BA in 1772 and his MA in 1775 having been elected to a Fellowship at Pembroke in 1773.

When William Pitt the Younger came up as an undergraduate to Pembroke at the age of 14 in 1773, Pretyman became his Tutor and friend, a relationship that would last a lifetime. Pitt's appointment as Prime Minister in December 1783 brought Pretyman to London as Pitt's private secretary in all but name. In 1787 Pretyman became Bishop of Lincoln, a post he held until 1820 when he was nominated Bishop of Winchester. He tended Pitt on his deathbed and undertook to produce a biography of Pitt.

Pretyman married Elizabeth (d. 1826), eldest daughter of Thomas Maltby of Germains, Buckinghamshire in 1784; they had three sons. In 1803 Pretyman inherited an estate in Riby, Lincolnshire from Marmaduke Tomline; Pretyman took the name Tomline from this point. Pretyman Tomline died at Kingston Hall, near Wimborne, Dorset, on 14 November 1827 and was buried in Winchester Cathedral on 28 November.