As a child Andreae went to Eton College where he was best friends with David Cameron. He then went on to Oxford University (Worcester College, 1985 – 88). Andreae completed his degree under difficult circumstances as he developed Hodgkin's lymphoma, a cancer of the lymph glands, and began an intensive course of chemotherapy and radiotherapy the day before his final exams began. He managed to sit some of his papers, and was awarded an upper second class degree. Giles lives with his wife Victoria, a children’s clothes designer, in Notting Hill, where he shares an office with writer Richard Curtis and radio and TV presenter Mariella Frostrup. He also has a seaside retreat in Dorset. Giles and Victoria have four children: Flinn, twins - Freya and Nat - and Jakson.

Purple Ronnie is a cartoon character used in greetings cards, T-shirts, and formerly for the Vimto soft drink campaign. He is like a stick man except he has a large oval body, with crudely-drawn stick-like limbs. He is usually shown as a comic poet, with occasional use of mild taboo language and topics.

In 2006 Andreae worked alongside Blunderbus Theatre Company (Nottinghamshire) to adapt his story Giraffes Can't Dance to bring it to stage. The company toured this story across the UK.