So what is a god exactly?
One of the big themes of The Elder Scrolls, especially The Elder Scrolls III: Morrowind, is that the line dividing gods from non-gods is pretty murky. And indeed, who is considered to be a god and who isn't has changed over the course of Tamriel's history.
For the sake of simplicity, I'll define a god as either:
- Someone who, at some point in time, achieves majority consensus from a culture that they are indeed a god.
- Someone who is directly referred to as a god in game narration (such as journal entries)
For instance, the majority of Imperial culture has at some point in time considered the Nine Divines and the Daedric Princes to be gods. And Mannimarco is called a god by the narrator in a Daggerfall cutscene. Therefore I will consider them to be gods for the purposes of this answer.
Former mortals who meet this criteria
Several Aldmeri ancestors
The Aldmer, precursors to the Altmer (High Elves) and Bosmer (Wood Elves), had several ancestors who were later worshiped as gods.
The religion of the people also changed because of this change in society: no longer did the Aldmer worship their own ancestors, but the ancestors of their [social] "betters." Auriel, Trinimac, Syrabane, and Phynaster are among the many ancestor spirits who became Gods.
–Pocket Guide to the Empire, 3rd Edition: "The Blessed Isle: Alinor and the Summersets", included with the Collectors Edition of Oblivion
These individuals lived during the Merethic Era, where history records are sparse, but we do have records of some of their lives as mortals (e.g. Syrabane aided a Colovian King).
Like Tiber Septim centuries later, Reman was an emperor who became worshiped as a god. It's a little unclear to me if he was worshiped as a god while he was living or after he died.
Culture god-hero of the Second Empire, Reman was the greatest hero of the Akaviri Trouble. Indeed, he convinced the invaders to help him build his own empire, and conquered all of Tamriel except for Morrowind. He instituted the rites of becoming Emperor, which included the ritual geas to the Amulet of Kings, a soulgem of immense power. His Dynasty was ended by the Dunmeri Morag Tong at the end of the first era. Also called the Worldly God.
–Varieties of Faith in the Empire, first appearing in Morrowind
Talos (a.k.a. Tiber Septim)
Tiber Septim was the emperor who united all of Tamriel under one empire. When he died, he ascended into godhood and was worshiped as equal to the Eight Divines.
As citizens of the Empire, all are of course familiar with the deeds of Emperor Tiber Septim. But it is the Emperor's ascent to godhood, as Talos, that is the subject of this work.
–The Talos Mistake, first appearing in Skyrim
Note that there are some, like the Thalmor, who dispute that Tiber Septim became a god. However, the Nerevarine (hero of Morrowind) is visited by Wulf, who is identified as an aspect of Talos, lending support to the notion that he did indeed become one.
The three members of the Tribunal (Vivec, Almalexia, and Sotha Sil) considered themselves to be gods, and were worshiped as such for centuries.
But Sotha Sil said to [Azura], "The old gods are cruel and arbitrary, and distant from the hopes and fears of mer. Your age is past. We are the new gods, born of the flesh, and wise and caring of the needs of our people. Spare us your threats and chiding, inconstant spirit. We are bold and fresh, and will not fear you."
–The Battle of Red Mountain, and the Rise and Fall of the Tribunal, first appearing in Morrowind
Note that later in the tale, Azura chides the Tribunal for deluding themselves into thinking that they are gods. Additionally after their decline, the "New Temple" relegates them from "gods" to "saints" (see The Reclamations: The Fall of the Tribunal and the Rise of the New Temple). One of the big themes of Morrowind is "what is a god?", so it's not surprising that this is debated. Still, they are widely recognized as gods during their reign.
Dagoth Ur and his minions
The details are a bit murky, given that Dagoth Ur was killed by the Tribunal once before, but he seems to have gained god-like power equivalent to that of the Tribunal.
[Nerevar's] trusted Tribunal - Almalexia, Sotha Sil, and Vivec - were found to have attained great, even god-like, power. His general, Dagoth Ur, originally thought killed at Red Mountain, was later found to have attained similar degree of power.
–Pocket Guide to the Empire, 3rd Edition: "The Temple: Morrowind", included with the Collectors Edition of Oblivion
When Dagoth Ur fought the Tribunal during their final pilgrimage to the Heart of Lorkhan, he claimed that he and his minions were gods.
Behold arrogant Tribunal! The power of the Heart flows through me and my minions. We are the gods now, growing stronger as you grow weaker.
–Dagoth Ur in "The Red Mountain", The Elder Scrolls: Legends
Dagoth Ur planned on having "god-priests" to worship Akulakhan (see below), so this corroborates that he brought others to apotheosis.
Establish the ancient heirs of House Dagoth as the god-priests of Akulakhan, and the Sixth House of Dagoth Ur as the dominant political power in Morrowind.
–Dagoth Ur's Plans, first appearing in Morrowind
Mannimarco (a.k.a. The King of Worms)
One of the six endings to The Elder Scrolls II: Daggerfall involves giving the Mantella to Mannimarco, who uses to become a god.
The Mantella is hurled from Aetherius, and although drawn to the empty chest of great Numidium, the will of the King of Worms commands it to his side. With this power, the King of Worms leaves his mortal frame and joins the ranks of the gods of Oblivion.
–King of Worms ending cutscene, Daggerfall
Due to shenanigans involving time moving non-linearly in an event called the Warp of the West, all six endings to Daggerfall happened simultaneously and are equally canon. Additionally, Where Were You When the Dragon Broke? about this event lists Mannimarco as "God of Worms, the Necromancers".
The Champion of Cyrodiil (a.k.a. The Hero of Kvatch)
As a result of The Shivering Isles expansion of The Elder Scrolls IV: Oblivion, the player character ascends to godhood equivalent to that of the other Daedra.
You now hold the mantle of madness, and Jyggalag is free to roam the voids of Oblivion once more. I will take my leave, and you will remain here, mortal. Mortal...? King? God? It seems uncertain. This Realm is yours. Perhaps you will grow to your station. Fare thee well, Sheogorath, Prince of Madness.
–The former Sheogorath, The Shivering Isles
Upon completing the penultimate quest in Oblivion, "Light the Dragonfires", you receive this journal entry, which leaves open the possibility that Martin Septim might have become a god.
Martin shattered the Amulet of Kings and transformed himself into an avatar of Akatosh, the Dragon God of Time. In dragon form he banished Mehrunes Dagon to Oblivion and ended the Oblivion Crisis, and then vanished. Whether he is dead, or has ascended to join his ancestor Tiber Septim, no one knows.
Back in the days of the Aldmer, Orgnum and others were exiled to Pyandonea, an island far to the south of Tamriel, and became the Maormer. Every so often they try to invade Summerset Isle, and every time it's led by King Orgnum himself, who appears younger each time.
Orgnum, their leader and self-styled "King," according to the legend, was a phenomenally wealthy Aldmer nobleman, who used his finances to launch a rebellion against the powers of the land. He and his followers were banished for this to a place separated from Aldmeris by an impenetrable mist, Pyandonea, "The Veil of Mist." This boundary proved so effective that the followers of Orgnum never again disturbed their former countrymen. The new Aldmeri homeland of Summerset, however, was not so lucky.
For much of Summerset's history, the Maormer have launched attacks against their sister child of Aldmeris. Every one of these battles have been led by Orgnum himself, who it seems is not only immortal, but grows more youthful by the century.
–Pocket Guide to the Empire, 3rd Edition: "Other Lands", included with the Collectors Edition of Oblivion
He is also said to be a "Serpent God" as part of Satakal, a Redguard religion.
The Maormer ruler is King Orgnum, a deathless wizard who is said to be the Serpent God of the Satakal.
–Pocket Guide to the Empire, 1st Edition: "The Wild Region", included with Redguard
Unfortunately, we don't have a lot of information about if his immortality and perpetual youth are indeed because he became a god.
Addendum: The Constructed Gods
While not "mortal" per se, it may be of interest to you that there were two "constructed gods": Numidium, "The Brass God", and Akulakhan, the "Second Numidium". The latter was powered by the Heart of Lorkhan (believed to be the literal heart of the god himself) and the former by the Mantella, a special soulgem designed to mimic the Heart of Lorkhan.