Sir Osbert Lancaster CBE (4 August 1908 – 27 July 1986) was an English cartoonist, architectural historian, stage designer and author. He was known for his cartoons in the British press, and for his lifelong work to inform the general public about good buildings and architectural heritage.

The only child of a prosperous family, Lancaster was educated at Charterhouse School and Lincoln College, Oxford; at both he was an undistinguished scholar. From an early age he was determined to be a professional artist and designer, and studied at leading art colleges in Oxford and London. While working as a contributor to The Architectural Review in the mid-1930s, Lancaster published the first of a series of books on architecture, aiming to simultaneously amuse the general reader and demystify the subject. Several of the terms he coined as labels for architectural styles have gained common usage, including "Pont Street Dutch" and "Stockbroker's Tudor", and his books have continued to be regarded as important works of reference on the subject.

In 1938 Lancaster was invited to contribute topical cartoons to The Daily Express. He introduced the single column-width cartoon popular in the French press but not until then seen in British papers. Between 1939 and his retirement in 1981 he drew about 10,000 of these "pocket cartoons", which made him a nationally known figure. He developed a cast of regular characters, led by his best-known creation, Maudie Littlehampton, through whom he expressed his views on the fashions, fads and political events of the day.

From his youth, Lancaster wanted to design for the theatre, and in 1951 he was commissioned to create costumes and scenery for a new ballet, Pineapple Poll. Between then and the early 1970s he designed new productions for the Royal Ballet, Glyndebourne, D'Oyly Carte, the Old Vic and the West End. His productivity declined in his later years, when his health began to fail. He died at his London home in Chelsea, aged 77. His diverse career, honoured by a knighthood in 1975, was celebrated by an exhibition at the Wallace Collection marking the centenary of his birth and titled Cartoons and Coronets: The Genius of Osbert Lancaster.

Lancaster was born at 79 Elgin Crescent in London in 1908, the only child of Robert Lancaster (1880–1917) and his wife, Clare Bracebridge, née Manger. His paternal grandfather, Sir William Lancaster, rose from modest beginnings to become the chief executive of the Prudential Assurance Company, Lord of the manor of East Winch, Norfolk, and a philanthropist in the field of education. Osbert's mother was an artist, known for her paintings of flowers, who had exhibited regularly at the Royal Academy; his father was a publisher, who volunteered for the army on the outbreak of the First World War, was commissioned as a second lieutenant in the Norfolk Regiment, and was killed at the Battle of Arras in April 1917.

Elgin Crescent, Notting Hill, where Lancaster was born and raised, was an upper-middle class area. The family maintained a staff of servants, including a cook and a nurse. Such was the mixed nature of London in the early years of the 20th century that a short distance away were the deprived and dangerous Notting Dale and the Portobello Road, where, as Lancaster recalled in his 1953 memoirs, it was said to be impossible for a well-dressed man to walk and emerge intact. From an early age Lancaster was aware of the variety of classes, nationalities, and social attitudes around him.

In 1918 Lancaster was sent to St Ronan's preparatory school, Worthing. The régime at the school leaned heavily towards sport, in which he was neither interested nor proficient. The headmaster, Stanley Harris, was a celebrated amateur footballer and occasional first class cricketer, but he was reasonably tolerant of Lancaster's disdain for games, and on the whole Lancaster enjoyed his time at the school. His education there was, he later commented, of more importance to him than anything he learned later in his school and university career. He left St Ronan's in 1921, aged thirteen, and went to Charterhouse, where his father and uncles had all been sent. There he was shocked by the bullying and bad language, but in addition to its sporty, philistine "bloods", the school had an intellectual and aesthetic tradition. Lancaster's biographer Richard Boston writes, "The hearty Baden-Powell, for example, was offset by Ralph Vaughan Williams and Robert Graves, while talented Carthusian artists had included Thackeray, Leech, Lovat Fraser and Max Beerbohm". The art master, P. J. ("Purple") Johnson, encouraged Lancaster, insisting that a sound technique was a prerequisite for effective self-expression in drawing or painting; in that respect the boy's time at the school was valuable, though otherwise the headmaster found him "irretrievably gauche ... a sad disappointment". Lancaster shared Beerbohm's view that being an old boy of the school was more pleasurable than being a pupil there.

At the age of seventeen Lancaster passed his final school examinations and gained entrance to Lincoln College, Oxford, to study history. He persuaded his mother to allow him to leave Charterhouse at once, giving him several months between school and university, during which he enrolled on a course of life classes at the Byam Shaw School of Art in London. In October 1926 he started at Oxford. There, as at Charterhouse, he found two camps in which some students chose to group themselves: the "hearties" presented themselves as aggressively heterosexual and anti-intellectual; the "aesthetes" had a largely homosexual membership. Lancaster followed his elder contemporary Kenneth Clark in being contentedly heterosexual but nonetheless one of the aesthetes, and he was accepted as a leading member of their set. He cultivated the image of an Edwardian dandy, with large moustache, a monocle and check suits, modelling his persona to a considerable degree on Beerbohm, whom he admired greatly. He also absorbed some characteristics of the Oxford don Maurice Bowra; Lancaster's friend James Lees-Milne commented, "Bowra's influence over Osbert was marked, to the extent that he adopted the guru's booming voice, explosive emphasis of certain words and phrases, and habit in conversation of regaling his audiences with rehearsed witticisms and gossip." Lancaster's undergraduate set included Stephen Spender, Randolph Churchill, and most importantly John Betjeman, who became a close friend and lifelong influence.