Though he lived most of his life in England and served in the British Army, he was refused a British passport in 1960, having been born outside Britain to an Irish father, Leo Milligan, who was born in Holborn Street Sligo. Milligan took Irish citizenship instead and never forgave the British Government.

He suffered from bipolar disorder for most of his life, having at least ten mental breakdowns. He was a strident campaigner on environmental matters, particularly arguing against unnecessary noise, such as the use of Muzak. During World War II he served as a signaller in the Royal Artillery. He saw action in North Africa and also Italy, where he was wounded and hospitalized for shell shock.

During most of the 1930s and early 1940s he performed as an amateur jazz vocalist and trumpeter both before and after being called-up, but even then he wrote and performed comedy sketches as part of concerts to entertain troops.

After his hospitalization he drifted through a number of rear-echelon military jobs, eventually ending up playing guitar with a jazz/comedy group called The Bill Hall Trio, at first in concert parties for the troops and, after the war, for a short time on stage. While he was with the Central Pool of Artists (a group, in his own words, "of bomb-happy squaddies") he began to write parodies of their mainstream plays, that displayed many of the key elements of what would become The Goon Show with Peter Sellers and Michael Bentine.

Milligan returned to England in the late 1940s and made a precarious living with the Hall trio and other similar acts mixing music and comedy, while attempting to break into the world of radio, both performing and scriptwriting. His first successes in this area were with material for the Derek Roy show, but he and some of his friends soon became involved with a much more unorthodox project - Crazy People, which rapidly mutated into The Goon Show.

Milligan's memoirs cover the years from 1939 to 1950 (essentially his call-up, war service, first breakdown, time spent entertaining in Italy, and return to the UK) in seven volumes.

He was the primary author of The Goon Show scripts (though many were written jointly with Eric Sykes and others) as well as a star performer, and is considered the father of modern British comedy, having inspired countless writers and performers with his work on The Goon Show and his own Q series, including Monty Python's Flying Circus. Writing a show a week affected his health greatly and caused him to have a series of nervous breakdowns. On one occasion, Peter Sellers had to lock his door against a potato-peeler-wielding Milligan; on another, Sellers and Harry Secombe broke into Milligan's dressing room, fearing he was suicidal. Eventually lithium was found to be the most effective treatment.

He also had a number of acting parts in theatre, film and television series; one of his last screen appearances was in the BBC dramatisation of Mervyn Peake's Gormenghast, and he was (almost inevitably) noted as an ad-libber. One of Spike's most famous ad-lib incidents occurred during a visit to Australia in the late 1960s. He was interviewed live-to-air and remained in the studio for the news broadcast that followed (read by Rod McNeil) during which Milligan constantly interjected, adding his own name to news items. As a result, he was banned from making any further live appearances on the ABC. The ABC also changed its national policy so that talent had to leave the studio after interviews were complete. A tape of the bulletin survives and has been included in an ABC Radio audio compilation, also on the BBC tribute CD, Vivat Milligna.

Milligan also wrote nonsense verse for children, the best of which is comparable with that of Lewis Carroll and Edward Lear, and (while depressed) serious poetry. He also wrote a very successful series of war memoirs, including Adolf Hitler: My Part in his Downfall (1971) and Rommel? Gunner Who? A Confrontation in the Desert (1974). He also wrote comedy songs, including Purple Aeroplane, which was a parody of The Beatles' song, Yellow Submarine.

After their retirement, Milligan's parents and his younger brother Desmond moved to Australia. His mother lived the rest of her life in the coastal village of Woy Woy on the New South Wales Central Coast, just north of Sydney; as a result, Spike became a regular visitor to Australia and made a number of radio and TV programmes there.

From the 1960s onwards Milligan was a regular correspondent with Robert Graves. Milligan's letters to Graves usually addressed a question to do with classical studies. The letters form part of Graves' bequest to St. John's College, Oxford.

In 1972, Milligan caused controversy by "liberating" a live shark from an art exhibition at the Hayward Gallery. In 1996, he successfully campaigned for the restoration of London's Elfin Oak.

The Prince of Wales was a noted fan, and Milligan caused a stir by calling him a "little grovelling bastard" on television in 1994. He later faxed the prince, saying "I suppose a knighthood is out of the question?". He was finally made an Honorary Knight Commander of the British Empire (KBE) (honorary because of his Irish citizenship) in 2000. He had been made an Honorary Commander of the British Empire (CBE) in 1992.

Milligan had three children with his first wife June Marlow: Laura, Seán and Síle. He had one daughter with his second wife Patricia Milligan: the actress Jane Milligan. He had no children with his third (and last) wife Shelagh Sinclair. The four children have recently collaborated with documentary makers on a new multi-platform program called I Told You I Was Ill: The Life and Legacy of Spike Milligan (2005) and web site, (see).

Even late in life, Milligan's black humour had not deserted him. After the death of friend Harry Secombe from cancer, he said, "I'm glad he died before me, because I didn't want him to sing at my funeral". A recording of Secombe singing was played at Milligan's memorial service. In a BBC poll in August 1999, Spike Milligan was voted the "funniest person of the last 1000 years".

He died from liver disease, at the age of 83, on February 27, 2002, at his home in Rye, East Sussex.