Farmer’s work includes designs for The Sleeping Beauty for the Munich Opera House, Anna Karenina and The Three Musketeers for The Australian Ballet, Giselle for Texas and Rio de Janeiro, Swan Lake for the Royal Winnipeg Ballet and Hong Kong Ballet, The Great Gatsby for Pittsburgh Ballet Theatre, The Nutcracker for Cincinnati Ballet Company, Les Sylphides for the Teatro Municipal, Rio de Janeiro, A Midsummer Night's Dream for the Hirofumi Inoue Ballet de Chambre in Tokyo and Balanchine's Theme and Variations and Divertimento No. 15 for Sadler's Wells (now Birmingham) Royal Ballet. He designed Anna Karenina for the Ballet de Santiago Chile and for the Norwegian Ballet, La Traviata for London City Ballet, Cinderella for the Cincinnati Ballet, Gisellefor the Star Dancers Ballet Foundation in Tokyo, Coppélia for the Portuguese National Ballet and also for the Inoue Ballet Foundation in Tokyo and The Nutcracker for Pittsburgh Ballet. He designed Kenneth MacMillan's Winter Dreams for The Royal Ballet and his Manon for the Australian, Vienna, Houston and Kirov Ballets. In 1991 he designed Peter Darrell's Tales of Hoffmann for the Hong Kong Ballet. In 1994 Farmer designed the sets and costumes for Houston Ballet’s production of Manon. This production entered the repertoire of the National Ballet of Canada in 1996. He has designed Les Sylphides and Coppélia and, in 1995, Ashton's Birthday Offering for the Birmingham Royal Ballet. He has also designed the full length Madame Butterfly for The Australian Ballet. In 1996, he designed Swan Lake for the Hong Kong Ballet, returning in 1997 to design their Nutcracker. He designed Swan Lake for the English National Ballet at the Royal Albert Hall, London and in 2011, a new production of The Nutcracker.
In 2005, Farmer created additional designs and recreated original designs, by Robin and Christopher Ironside, for Sylvia, a co-production between American Ballet Theatre and The Royal Ballet.
Farmer has collaborated on numerous occasions with Robert Cohan and Robert North for London Contemporary Dance Theatre. These include Cohan's Stages, No-Man's Land, Sky, Consolations of the Rising Moon, Stone Garden, In Memory, Metamorphoses and Crescendo and North's Meeting and Parting, David and Goliath and Troy Game.
His drama work includes The Night of the Iguana, at the Savoy Theatre, Man and Superman at the Vaudeville, Kean at the Globe Theatre, What Every Woman Knows at the Albery and A Woman of No Importance for Chichester Festival Theatre.
Farmer is also a prolific book illustrator and a successful painter, who has had numerous one-man exhibitions of his work.