This painting hanging in the staircase-pavilion of the Wren Library is a copy of 1830 after Sir Thomas Lawrence’s portrait in the National Portrait Gallery, London. It was painted at the time of Lamb’s marriage to Lady Caroline Ponsonby in 1805. The marriage was unhappy, marred by infidelities on both sides, though most notoriously, Caroline’s affair with Lord Byron in 1812-13.
Melbourne first entered parliament in 1806 as member for Leominster and was appointed Home Secretary in the new Whig government in November 1830. Following the resignation of Lord Grey (also an alumnus of Trinity), Melbourne was appointed Prime Minister. Dismissed six months later by King William IV, he returned as Prime Minister in April 1835 until 1841.
The appearance of Trinity’s portrait of William Lamb, acquired in 1908, has been greatly improved by recent conservation and cleaning.
Its neighbour at the bottom of the Wren Staircase, is a portrait of another Trinity Prime Minister – Stanley Baldwin (1867-1947) – in his University of Cambridge Chancellor’s robes. It was painted by Sir Walter Thomas Monnington in 1932 and is currently away taking its turn at the conservators.
Trinity College Library, Cambridge 
Thank you so much for sharing this ! Especially the before and after cleaning and restoration.