| Final | time | | 1. Wyndham Halswelle |  Great Btitain | 50.0s | | -. William Robbins |  United States | DNS | | -. John Taylor | United States | DNS | | -. John Carpenter | United States | DSQ |
| First Final Thursday, 23rd July | time | | 1. John Carpenter | United States | 47.8s | | 2. Wyndham Halswelle | United States | Unknown | | 3. William Robbins | United States | Unknown | | 4. John Taylor | United States | Unknown |
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| | | Venue: White City Stadium - track of 536.45 metres=1⁄3 mile. | | 37 competitors from 10 nations. | | World Record: 47.8s - Maxie Long, New York (USA), September 29, 1900 | | Olympic Record: 49.2s - Harry Hillman, St. Louis (USA), August 29, 1904 |
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| It was the most controversial event of the London Games. The final resulted in the disqualification of American runner John Carpenter, who had blocked British athlete Wyndham Halswelle in a manoeuvre that was legal under American rules but prohibited by the British rules under which the race was run. As part of the disqualification of Carpenter, a second final race was ordered. Halswelle was to face the other two finalists again. These men, William Robbins and John Taylor, were both Americans and decided not to contest the repeat of the final to protest against the judges' decision. Halswelle was thus the only medallist in the 400 metres. |
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Semifinals - Wednesday, 22th July. | | Winner of each of 4 Semis advanced to Final. |
| Semi 1 | time | | 1. John Carpenter | USA | 49.4s | | 2. Charles Davies | GBR | 49.8s | | 3. Ned Merriam | USA | Unknown | | 4. G. W. Young | GBR | Unknown |
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| Semi 2 | time | | 1. Wyndham Halswelle | GBR | 48.4s OR | | 2. Edwin Montague | GBR | 49.8s | | 3. George Nicol | GBR | Unknown | | 4. William C. Prout | USA | Unknown |
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| Semi 3 | time | | 1. John Taylor | USA | 49.8s | | 2. Horace Ramey | USA | 50.5s | | 3. Edward Ryle | GBR | Unknown | | 4. Georges Malfait |  FRA | Unknown |
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| Semi 4 | time | | 1. William Robbins | USA | 49.0s | | 2. Louis Sebert |  CAN | 49.5s | | 3. John Atlee | USA | Unknown |
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Heats - Tuesday, 21st July. | | Winners of each of 16 Heats advanced to Semifinals. |
| Heat 1 | time | | 1. Edwin Montague | GBR | 50.2s | | 2. Paul Pilgrim | USA | 51.4s |
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| Heat 2 | time | | cancelled as there were no starters. |
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| Heat 3 | time | | 1. Edward Ryle | GBR | walkover |
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| Heat 4 | time | | 1. John Taylor | USA | 50.8s | | 2. Roberto Penna |  ITA | 52.4s | | 3. Sven Låftman |  SWE | Unknown |
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| Heat 5 | time | | 1. George Nicol | GBR | 50.8s | | 2. Oscar Guttormsen |  NOR | 52.4s |
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| Heat 6 | time | | 1. Georges Malfait | FRA | 50.0s | | 2. Donald Buddo | CAN | 51.2s |
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| Heat 7 | time | | 1. William Robbins | USA | 50.4s | | 2. József Nagy |  HUN | 51.1s | | 3. Noel Godfrey Chavasse | GBR | Unknown | | 4. Victor Henny |  NED | Unknown |
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| Heat 8 | time | | 1. William C. Prout | USA | 50.4s | | 2. Christopher Maude Chavasse | GBR | 50.7s |
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| Heat 9 | time | | 1. Horace Ramey | USA | 51.0s | | 2. Arthur Astley | GBR | Unknown |
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| Heat 10 | time | | 1. Louis Sebert |  Can | 50.2s | | 2. Massimo Cartasegna | ITA | 52.7s | | -. Victor Jacquemin |  BEL | DNF |
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| Heat 11 | time | | 1. John Atlee | USA | 50.4s | | 2. Alan Patterson | GBR | 50.6s | | 3. Giuseppe Tarella | ITA | Unknown |
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| Heat 12 | time | | 1. Charles Davies | GBR | 50.4s | | 2. Cornelis den Held | NED | 51.0s |
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| Heat 13 | time | | 1. Ned Merriam | USA | 52.2s | | 2. R. C. Robb | GBR | 52.5s |
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| Heat 14 | time | | 1. John Carpenter | USA | 49.8s | | 2. Otto Trieloff |  GER | 50.9s | | 3. Arvid Ringstrand | SWE | Unknown | | 4. Henk van der Wal | NED | Unknown |
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| Heat 15 | time | | 1. Wyndham Halswelle | GBR | 49.4s | | 2. Frederick de Selding | USA | 50.8s | | 3. Bram Evers | NED | Unknown |
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| Heat 16 | time | | 1. G. W. Young | GBR | 52.4s | | 2. Jacobus Hoogveld | NED | (54.3s) |
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