Bill Buchanan on Day 7
|Portrayed by||James Morrison|
|Seasons||4, 5, 6, 7|
|First appearance||Day 4: 9:00 PM – 10:00 PM|
|Last appearance||Day 7: 8:00 PM – 9:00 PM|
Bill Buchanan has a Bachelor Degree in Arts from Brown University. He began his intelligence career as an intelligence analyst at CTU: New York. Later, he became a Senor Agent in the CTU: Seattle Office. After that, he became Associate Special Agent at the CTU Division Office in Los Angeles.
 Day 4
After Erin Driscoll resigns as Special Agent in Charge of CTU: Los Angeles, Bill is sent is from Division to help CTU through the day's crisis. Soon after Bill Buchanan is sent to CTU, Michelle Dessler is sent to take over. We learn that Dessler and Buchanan had been romantically involved when they both worked at Division after she divorced Tony Almeida. Although their relationship is over during Day 4, there is still feelings between them. Buchanan helps Dessler and others successfully thwart the worst of the terrorist attacks during Day 4.
 Day 5
At the beginning of Day 5, Bill Buchanan is now the Special Agent in Charge of CTU: Los Angeles. Although he is troubled by Michelle Dessler's death, he shows great leadership. Despite CTU being perfectly operational, Lynn McGill is sent from Division to help out. He continually contradicts Buchanan and also takes over his office. McGill feels that everyone is out to get him, so he throws Bill Buchanan in holding in order to show his authority. As the staff begins to realize that McGill is incompetent, they help to reinstate Bill Buchanan.
After CTU is hit by a devastating nerve gas attack, more bureaucrats from Homeland Security come to CTU to absorb it. Along with many other competent CTU workers, Buchanan is fired and sent home. Karen Hayes takes over his position. Buchanan is hurt when Audrey Raines signs a document that places the blame from the day's events on Buchanan. He does not know that Audrey signed the document in order to keep Chloe O'Brian on staff.
Later, Jack and Wayne Palmer come to Bill's house. Bill takes Wayne to a safe location. Later, Chloe O'Brian comes to his door and they set up a makeshift CTU. She was being kept in CTU detention but was able to escape. Karen Hayes, who is now becoming very skeptical about the President, calls Buchanan and warns him a CTU team is coming to his house. Bill leaves evidence that Chloe was there in order to be brought back for an "interrogation" by Karen Hayes. Bill brings her up to speed with him and Chloe. They spend the rest of the day trying to help Jack.
When they learn that Vladimir Bierko has escaped and that he is on his way to a submarine, they take the necessary actions. After Jack kills Bierko and Christopher Henderson, the main crisis is over. Later, Buchanan helps contact the Attorney General to remove Charles Logan from power.
As the day is coming to a close, Bill begins to leave thinking he has been relieved of duty. Karen tells him that he is still needed at CTU. Then, Bill and Karen make plans to go out sometime. Buchanan then hands a photograph of Edgar and Chloe together that he found in Edgar's possessions.
 Day 6
At the beginning of Day 6, Buchanan is married to Karen Hayes, who is now the National Security Adviser. Buchanan is still the head of CTU.
 Day 7
- Bill had the longest term as the Special Agent in Charge of CTU: Los Angeles in the series.
- Bill was the tenth main character to be killed in the series.
- Bill received a silent clock after his death.