First Time Lord (Colony in Space)

A Time Lord , by one account clarified to be the First Time Lord, (PROSE: Doctor Who and the Doomsday Weapon) was on a figure of authority during the Third Doctor's exile on Earth. He often received important reports. (TV: Colony in Space) By one account, as First Time Lord, he was the ruler of the Time Lords, alongside the two other members of the High Tribunal. (PROSE: Doctor Who and the Doomsday Weapon)

By another account, the Time Lord in charge of the Uxarieus mission was an incarnation of Deliavatsud. (PROSE: A Sourcebook for Field Agents)

Origins
As the First Time Lord, this Time Lord was a regeneration of the First Time Lord who presided over the Doctor's trial. (PROSE: Doctor Who and the Doomsday Weapon)

Uxarieus mission
When a report on the Doomsday Weapon went missing from the Time Lords' files, he and the other Time Lords were convinced that the Master had taken it. Alongside the Second Time Lord and Third Time Lord, he decided to send the Third Doctor to the planet Uxarieus in the year 2472 to investigate. (TV: Colony in Space)

One account indicates that these Time Lords were not those who sent the Third Doctor on his later missions. While meeting with the Three, Adelphi noted that their "esteemed... associates" who sent the Doctor to Exarius were severely punished for breaking the non-interference policy. (PROSE: Legacy)

Fate
Further accounts of Gallifrey would contradict the account that it was ruled by First Time Lord, (TV: The Three Doctors, et al.) although some indicated the High Tribunal continued to operate, even presiding the Sixth Doctor's trial. (PROSE: The Quantum Archangel, Synthespians™)

Although no account directly connects this First Time Lord with him, the President who sent the Third Doctor on his final mission remembered being the previous First Time Lord, opening the possibility that this First Time Lord may have regenerated into this President. (PROSE: The Three Doctors)