President of Earth

The President of Earth was a title held by one person at a time by at least the 26th century. Both Earth and the broader Earth Empire were ruled by this elected figure.

He or she did not, however, have full control over every aspect of the Empire. In purely military matters, the president was unable to overrule the head of the military without the backing of the Earth Senate. Also, the President of Earth had limited control over the affairs of so-called "Dominions", like Sirius IV. These former colonies were given broad autonomy to handle their own affairs, over which the president had no control.

In 2540 the president came under great pressure to declare war on the Draconians, especially from General Williams, but was able to maintain peace thanks to the the Doctor. (DW: Frontier in Space)

At some point Duncan Levinson was president of Earth. (BFC: Scorpius)
 * According to the novelisation of Frontier in Space'', the president led a democracy, and had to submit to periodic election cycles in which various candidates argued their positions in order to win popular approval. On the other hand, the novel also describes Earth Security as a shadowy police organization which could hold troublemakers without trial.  Both the novel and the serial agree that in extreme cases, the president had the power to remove political rivals to the Lunar Penal Colony for the rest of their lives.  The position thus had elements of democracy and dictatorship about it.