Born in London, he belonged to a family claiming descent from the father of St Thomas a Becket. His elder brother, Sir William a Beckett (1806-1869), became chief justice of Victoria, Australia. Gilbert Abbott a Beckett was educated at Westminster School, and was called to the bar at Gray's Inn in 1841. He edited Figaro in London, and was one of the original staff of Punch and a contributor until his death. He was an active journalist on The Times and The Morning Herald, contributed a series of light articles to The Illustrated London News, conducted in 1846 The Almanack of the Month and found time to produce some fifty or sixty plays, among them dramatized versions of Charles Dickens' shorter stories in collaboration with Mark Lemon. As poor-law commissioner he presented a valuable report to the home secretary regarding scandals in connection with the Andover Union, and in 1849 he became a metropolitan police magistrate. He died in Boulogne, France of typhoid.
He was the father of two other Victorian writers, Gilbert Arthur a Beckett and Arthur William a Beckett.