William was born in St Pancras, London, England, the only child of an unmarried mother, and raised primarily by his aunt Bessie (Would, 208). He never discovered the identity of his father (whose particulars are left blank on the birth certificate) and, despite efforts made in later years, was never able to trace him. Known to friends as Billy, he was educated at home and at Imperial Service College. After training as a jockey, and boxer, he studied at the Italia Conti Academy and entered the theatre in 1924 working under Frank Benson. The first of more than sixty film appearances was Say It With Music in 1932.

William Hartnell usually played comic characters, until 1944 when he was cast in the robust role of sergeant Ned Fletcher in The Way Ahead. From then on his career was defined by playing mainly policemen, soldiers, and thugs — although he was noted for his ability to bring complexity to such roles, for example his widely praised performance as Dallow in Brighton Rock.

In 1958 he topped the bill in the first Carry On film, Carry On Sergeant playing Sergeant Grimshaw, and in 1963 he appeared as a town councillor in the Boulting Brothers' film Heavens Above! with Peter Sellers. William Hartnell also appeared as Will Buckley in the film The Mouse That Roared in 1955 (again with Sellers).

His first regular role on television was in The Army Game from 1957–1961. In 1963 he appeared in a supporting role in the film version of This Sporting Life, giving a sensitive performance as an ageing rugby league talent scout known as 'Dad'.

During the 1960s he lived in a house on Eel Pie Island, a small island in the middle of the River Thames near Twickenham in London.

His performance in This Sporting Life was noted by Verity Lambert, the producer who was setting up a new science-fiction television series for the BBC, Doctor Who, who then offered him the title role. Although he was initially uncertain, Lambert and director Waris Hussein convinced him to take the part and it became the character for which he gained the highest profile and is now most widely remembered. William came to relish particularly the attention and affection playing the character brought him from children, and he became very fond of the role which also earned him a regular salary of £315 per episode by 1966 (equivalent to £4,050.90 in modern currency). In comparison, his co-stars Anneke Wills and Michael Craze earned £68 and £52 per episode at the same time.

William Hartnell suffered a bereavement in 1965 whilst working on The Myth Makers: his Aunt Bessie, who had looked after him during his troubled childhood, died. The tight production schedules prevented him from taking time off to attend her funeral.

According to some colleagues on Doctor Who, he could be a difficult person to work with, although others, notably actors Peter Purves and William Russell, and producer Verity Lambert, speak glowingly of him after more than forty years. His poor health (arteriosclerosis) as well as poor relations with the new production team on the series following the departure of Lambert, ultimately led him to leave Doctor Who in 1966.

Some commentators now contend that reports of William Hartnell's illness were subsequently exaggerated by Lambert's successors in the role of producer, John Wiles and Innes Lloyd, to justify a desire (ultimately successful) to remove the expensive actor from the series. Others suggest that it was a mutual decision between William Hartnell and the production team that he should leave the programme. However, he claimed in later life that he did not want to leave the series, writing, in an oft-quoted letter, "I didn't willingly give up the part". Suggestions that his health was failing him are seemingly contradicted by his return to demanding theatre work almost immediately upon leaving Doctor Who. He also made television guest appearances across the late 1960s including in No Hiding Place.

William Hartnell reprised his most famous role in the 10th Anniversary story The Three Doctors (made in 1972, broadcast 1973) with the help of cue cards, but appeared only in pre-filmed inserts seen on video screens.His health had grown progressively worse in the early 1970s and in December 1974 he was admitted to hospital permanently. In early 1975 he suffered a series of strokes brought on by cerebrovascular disease and died in his sleep of heart failure on April 23, 1975 at the age of 67. His death was reported on the BBC News and a clip of the Doctor in the TARDIS from the end of The OK Corral, the final episode of The Gunfighters, was shown.

A clip of his scene from the end of the serial The Dalek Invasion of Earth (1964) was used as a pre-credits sequence for the 20th anniversary story The Five Doctors (1983), although another actor, Richard Hurndall, played the role of the First Doctor for the rest of the episode.

William Hartnell was married with one daughter. Other biographical information about William Hartnell is hard to substantiate because of conflicting information from various sources. He, himself, gave accounts of his birth and upbringing which seem to differ from verifiable facts, and the only published biography of him is by his granddaughter, Jessica Carney. Although criticised by some as a hagiography, Jessica's "Who's There?" does refer to these difficulties and makes it clear that a great deal of research has been done, drawing from primary sources, as well as his family's own extensive archive. Notwithstanding an often negative view of its subject, the family link with the author makes some critics view this work as biased.

Film and TV

This Sporting Life

This Sporting Life (1963) .... 'Dad' Johnson
Heavens Above! (1963) .... Major Fowler
To Have and to Hold (1963) .... Inspector Roberts
... aka Edgar Wallace Mysteries: To Have and to Hold (UK: series title)
The World Ten Times Over (1963) .... Dad
... aka Pussycat Alley
"Ghost Squad" .... Fred (1 episode, 1961)
... aka G.S.5 (UK: third season title)
- High Wire (1961) TV Episode .... Fred
"Kraft Mystery Theater" (1 episode, 1961)
- The Desperate Men (1961) TV Episode
And the Same to You (1960) .... Wally Burton
Jackpot (1960) .... Supt. Frawley
Piccadilly Third Stop (1960) .... Colonel

The Mouse That Roared

The Mouse That Roared (1959) .... Will Buckley
Shake Hands with the Devil (1959) .... Sergeant Jenkins
"The Flying Doctor" .... Abe McKeller (1 episode, 1959)
- The Changing Plain (1959) TV Episode .... Abe McKeller
Strictly Confidential (1959) .... Grimshaw

The Desperate Man

The Desperate Man (1959) .... Smith
"Dial 999" .... Jeff Richards / Joss Crawford (2 episodes, 1959)
- 50,000 Hands (1959) TV Episode .... Jeff Richards
- The Killing Job (1959) TV Episode .... Joss Crawford
The Night We Dropped a Clanger (1959) .... Sgt. Bright
... aka Make Mine a Double
Carry on Sergeant (1958) .... Sergeant Grimshawe
On the Run (1958) .... Tom Casey

The Army Game

"The Army Game" (1957) TV Series .... CSM Bullimore (1957-1958, 1960-1961) (unknown episodes)
Date with Disaster (1957) .... Tracy
The Hypnotist (1957) .... Inspector Ross
... aka Scotland Yard Dragnet (USA)
Hell Drivers (1957) .... Cartley, Hawlett Manager
... aka Hard Drivers

Yangtse Incident: The Story of H.M.S. Amethyst (1957) .... Leading Seaman Frank
... aka Battle Hell (USA)
... aka Escape of the Amethyst
... aka Their Greatest Glory
... aka Yangtze Incident
Doublecross (1956) .... Whiteway
Private's Progress (1956) .... Sgt. Sutton
Tons of Trouble (1956) .... Bert
"London Playhouse" .... Kenyon (1 episode, 1955)
- The Inward Eye (1955) TV Episode .... Kenyon
Footsteps in the Fog (1955) .... Herbert Moresby
Josephine and Men (1955) .... Inspector Parsons
Will Any Gentleman...? (1953) .... Detective Inspector Martin
The Holly and the Ivy (1952) .... Company Sergeant Major
The Pickwick Papers (1952) .... Irate Cabman
The Ringer (1952) .... Sam Hackett

The Magic Box

The Magic Box (1951) .... Recruiting Sergeant
The Dark Man (1951) .... The Police Superintendant
... aka Jeffrey Dell's The Dark Man (USA: complete title)
Double Confession (1950) .... Charlie Durham
The Lost People (1949) .... Barnes
Now Barabbas (1949) .... Warder Jackson
... aka Now Barabbas Was a Robber
Escape (1948) .... Inspector Harris
Temptation Harbour (1947) .... Jim Brown
... aka Temptation Harbor (USA)

Odd Man Out

Odd Man Out (1947) .... Fencie,the barman
... aka Gang War

Brighton Rock

Brighton Rock (1947) .... Dallow, henchman
... aka Young Scarface (USA)
Appointment with Crime (1946) .... Leo Martin
Murder in Reverse (1945) .... Tom Masterick
... aka Query (USA)

The Way Ahead

The Way Ahead (1944) (as Billy Hartnell) .... Sgt. Ned Fletcher
... aka The Immortal Battalion (USA: cut version)
Headline (1944) .... Dell

Strawberry Roan

Strawberry Roan (1944) .... Chris Lowe
The Agitator (1944) (as Billy Hartnell) .... Peter Pettinger
Suspected Person (1943) .... Saunders
The Dark Tower (1943) .... Towers

The Bells Go Down

The Bells Go Down (1943) (as Billy Hartnell) .... Brooks
Sabotage at Sea (1942) .... Jacob Digby
The Goose Steps Out (1942) (uncredited) .... German Officer At Station
They Flew Alone (1942) (as Billy Hartnell) .... Scotty, Airman Handing Weather Report To Amy
... aka Wings and the Woman (USA)
Flying Fortress (1942) (uncredited) .... Gaylord Parker, English cabbie
The Peterville Diamond (1942) .... Joseph
Freedom Radio (1941) (uncredited) .... Radio Location Aerial Operator
... aka A Voice in the Night (USA)
They Came by Night (1940) (uncredited) (unconfirmed) .... Undetermined Role
Murder Will Out (1940) (as Billy Hartnell) .... Dick
Too Dangerous to Live (1939) (as Billy Hartnell) .... Bit Part
They Drive by Night (1938) (as Billy Hartnell) .... Bus Conductor
Farewell Again (1937) (uncredited) .... Bit Role
... aka Troopship (USA)
Midnight at Madame Tussauds (1936) (as Billy Hartnell) .... Stubbs
... aka Midnight at the Wax Museum (USA)
Parisian Life (1936) (as Billy Hartnell)
Nothing Like Publicity (1936) (as Billy Hartnell) .... Pat Spencer
While Parents Sleep (1935) .... George
The Guv'nor (1935) (uncredited) .... Bit Role
... aka Mister Hobo (USA)
Seeing Is Believing (1934) .... Ronald Gibson
The Perfect Flaw (1934) .... Vickers
Swinging the Lead (1934) .... Freddy Fordum
Follow the Lady (1933) .... Mike Martindale
The Lure (1933) .... Billy
I'm an Explosive (1933) .... Edward Whimperley
Say It with Music (1932)