Unless you want to count David Rice Atchison (which you probably shouldn't), or the fictional Glen Allen Walken played by John Goodman on The West Wing, then yes, Ford is the only one.
Theoretically, a third person can become President if both offices become vacant. This has never happened. Well, except possibly once in 1849. Some people claim that Atchison (president pro tempore of the Senate at the time) was technically President for 24 hours across March 4-5. Most scholars dismiss this.
Absent that, in order for someone not elected President or Vice-President to become President, three things have to happen in a four-year span:
The office of Vice-president has to become vacant, either because the VP died, or resigned, or succeeded to President because the President died or resigned. This has happened 14 times through American history.
The President has to appoint someone to fill the vacancy, and Congress has to confirm this person.
Finally, the President has to leave office and the replacement Vice-President has to succeed.
It's that second bullet that's the kicker.
Before the 25th Amendment was ratified in 1968, there was no provision for filling a Vice-Presidential vacancy. The Constitution gives Congress the power to enact legislation to determine the succession order, and it has done this three times: in 1792, in 1886, and in 1947. Each time, however, the statute only provided for what happened when both offices were empty. The 25th Amendment, however, gave the President the power to appoint a new Vice-President, with Congressional confirmation. And the (unamended) Constitution does not provide for filling a vacancy.
Since 1968, this has happened only twice:
- Nixon appointed Ford after Agnew resigned.
- Ford appointed Nelson Rockefeller after succeeding to the Presidency. Both served out their term (Ford lost the 1976 election).
All Presidents and Vice-Presidents since then have served out their terms.
Congress may by Law provide for the Case of Removal, Death, Resignation or Inability, both of the President and Vice President, declaring what Officer shall then act as President, and such Officer shall act accordingly, until the Disability be removed, or a President shall be elected.
United States Constitution, Article II, Section I
And be it further enacted, That in case of removal, death, resignation or inability both of the President and Vice President of the United States, the President of the Senate pro tempore, and in case there shali be no President of the Senate, then the Speaker of the House of Representatives, for the time being shall act as President of the United States until the disability be removed or a President shall be elected.
SEC. 10. And be it further enacted, That whenever the offices of President and Vice President shall both become vacant, the Secretary of State shall forthwith cause a notification thereof to be made to the executive of every state, and shall also cause the same to be published in at least one of the newspapers printed in each state, specifying that electors of the President of the United States shall be appointed or chosen in'the several states within thirty-four days preceding the first Wednesday in December then next ensuing....
Presidential Succession Act, 1792
That in case of removal, death, President and resignation, or inability of both the President and Vice-President of Vice-President. the United States, the Secretary of State, or if there be none,or in case of his removal, death, resignation, or inability, then the Secretary of the Treasury, or if...
Presidential Succession Act, 1886
if, by reason of death, resignation removal from office, inability, or failure to qualify, there is neither a President nor Vice President to discharge the powers and duties of the office of President, then the Speaker of the House of Representatives shall, upon his resignation as Speaker and as Representative in Congress, act as President....
Presidential Succession Act, 1947
Whenever there is a vacancy in the office of the Vice President, the President shall nominate a Vice President who shall take office upon confirmation by a majority vote of both Houses of Congress.
Twenty-Fifth Amendment, section 2