CONTROL apparently had five chiefs from its formation to the end of the timespan being chronicled
1. Admiral Harold Hargrade (c.1930-1946). The start date is inferred from the Admiral's 1967 reference to the president as "Herbert", suggesting that he had been used to working with President Herbert Hoover. His retirement date is on his card in the CONTROL files. [Episode #59: "A Man Called Smart, Part 2".]
2. Unknown (1946-1965?). There could of course have been more that one chief during this period but given J. Edgar Hoover's 37-year tenure as Director of the FBI a single 19 year term is not out of the question.
3. Thaddeus a.k.a. Harold Clark (1965?-1971). The best-known chief of CONTROL and the one to whom the words "the Chief" refer on this wiki unless otherwise indicated. The 1965 date is inferred from the greeting he and Max exchange in the first episode - "Good to see you again" - suggesting that they had not seen each other for some time. Only a few episodes later [#8: "The Day Smart Turned Chicken"] it is stated that Max has worked on "over a hundred" cases so this is not his first assignment. It is therefore speculated that it is Thaddeus's first mission as Chief. It is also known that he was an instructor at the CONTROL Training School when Max was a student [Episode #130: "The Mess of Adrian Listenger] so he could not have succeeded Admiral Hargrade directly and it also ties in with Max's tendency to address him as "Sir" in this first outing. The 1971 date is foreshadowed in 1968 when he states that he only has three years to go until retirement [Episode #76: "The Little Black Book, Part 1"].
4. Unknown (1971-1974). Again, there could have been more than one chief during this period. Alternatively, Thaddeus could have stayed on beyond his expected retirement. 1974 is the date of CONTROL's deactivation ["15 years ago in "Get Smart, Again!"].
5. Maxwell Smart (1989-1995?). CONTROL's ad hoc revival with Max as leader to face an imminent KAOS threat takes place in 1989 [TV Move: "Get Smart, Again!".] It is assumed that the permant reactivation [as seen in the Get Smart (1995) series] occurred shortly after. It is worth noting that Max would have turned 65 in 1995 but further speculation on his retirement and successors is probably best left in the realm of fan fiction.