The third actor to play the Doctor on BBC's Doctor Who, Jon Pertwee, ushered in some changes to the series. Not only did viewers start to see more influence on the Doctor from other media of the time, but his era also saw the first color broadcast of the show. Perhaps this is why Pertwee's portrayal of the Third Doctor became so iconic -- and why so many were sad to see him leave the role in 1974.
Pertwee joined Doctor Who in 1970, taking over the role from Patrick Troughton. He had been shortlisted for the role before producer Peter Bryant offered it to him, perhaps after working with previous Doctor William Hartnell in an earlier film project Will Any Gentleman...? Regardless, Pertwee brought with him an air of action and a fresh take on the Doctor.
The Third Doctor was a departure from both of his earlier incarnations. For the first time, the Doctor's design was influenced by the release of the James Bond movies. This resulted in a focus on fancy clothes and cars, as well as the Doctor becoming a man of action. But Pertwee was also encouraged to play the Doctor as himself, without putting on voices or disguises, a sharp turn from a previous focus on character work.
After playing the Doctor for four years, Pertwee announced he would be leaving the role in early 1974. He cited many of the common reasons for departing Doctor Who, including wanting to avoid typecasting after playing the same notorious role for four years. Pertwee also had other roles to focus on. While he had been active in other projects even while playing the Doctor, Pertwee was particularly interested in resuming his stage career.
Another factor in Pertwee's departure, however, was the death of his friend Roger Delgado, who played the Master on Doctor Who. Katy Manning, who played companion Jo Grant, and producer Barry Letts had also left, adding to Pertwee's feeling that the band breaking up. The high turnaround coupled with the departure of friends seemingly created an environment where Pertwee felt ready to leave Doctor Who.
Other sources have cited reasons for Pertwee's departure that are harder to substantiate. It's possible that he asked for a raise and was denied, leading to his exit -- while there's no hard evidence to support this theory, there are anecdotes suggesting later Doctor actors teased Pertwee about how little he was paid. Other factors may have included Pertwee's back pain, which was the result of a war injury and led to a spinal operation in 1994. Whether this had any impact on his decision to leave Doctor Who, however, is unknown.
Jon Pertwee brought a sense of class and adventure to the Doctor and stayed on longer than either of the earlier actors who brought the character to life. His time also brought to life a number of standard Doctor Who characters, including the infamous Master. Whether he left solely because he felt it time to go or was influenced by health and money, Pertwee left a legacy of the Doctor as an action hero, unafraid to "reverse the polarity of the neutron flow."