(His physical appearance and attitude toward war vary between continuities, though his color scheme has stayed surprisingly consistent. See below:)
Marvel Comics continuity
Sentinel Prime is mentioned frequently in the recitation of Primes before Optimus, but little or nothing is known of him.
- (Note: The retelling of Unicron's origin in the UK comics included a brief glimpse of a dying Matrix bearer passing the Matrix of Leadership to Optimus Prime. However, as he is unnamed, the only way we can presume this being is Sentinel Prime is because, as we are told later, Sentinel Prime was the leader before Optimus. See image to the left.)
Dreamwave comics continuity
- Sentinel Prime's biggest shortcoming is that he leads with his head, a peacetime leader in a time of war. His successor, Optimus Prime, leads with his heart, backed by his fists— a much more suitable combination.
Sentinel Prime was Autobot leader for the first 500,000 years of the Great War. More physically massive than Optimus Prime, Sentinel Prime's armor was made of trithyllium alloy impregnated with irradiated carbon fibers, unbreakable short of a precisely-aimed point-blank shot.
Sentinel Prime's primary weapons were his swiveling shoulder cannons, capable of firing a variety of specialized payloads including concussive and shrapnel-needle shells, and gyro-inhibiting and magnetic inducer shells. (Disruptive warheads which foul an opponent's systems.) He was also armed with a solar-energy rifle and an energon-charged sword.
Sentinel Prime used an electro-disrupter, a device capable of projecting several copies of his image, to disorient his opponents in battle. When interrupted, electro-disrupter images de-rez, leaving a visible path back to the real projector. Because using direct attacks would reveal which image was the real Sentinel Prime, he must limit himself to indirect weapons like shrapnel shells when using it.
Sentinel Prime's only physical weakness was his shoulder-cannons. A direct hit down their barrel could cause the ammunition inside to explode, causing catastrophic damage. To minimize that risk, he swivels the cannons to his back except when about to fire. 
The War Within
8.2 million years ago, Sentinel Prime went to reason with Megatron, seeking to reach him with sermon and doctrine. He failed and was tortured and destroyed by Megatron. Megatron was chagrined to discover that Sentinel Prime (suspecting he may be killed) had not brought the Matrix with him into battle.
The Keepers Trilogy
The Matrix used a vision of Sentinel Prime to strengthen Optimus Prime's resolve during a battle with Megatron by confronting him with the claim that Megatron had chosen Optimus to be Prime, knowing that he was weak. Megatron's alleged influence over the Prime selection process was never substantiated.
The Keepers subsequently created a robotic duplicate of Sentinel Prime to confuse Optimus and undermine his confidence. Though possessing all of Sentinel Prime's considerable abilities, Optimus eventually realized that the duplicate had been created from his own memories of Sentinel Prime, and was incapable of doing anything new. Optimus destroyed the duplicate of Sentinel Prime, having largely overcome his own doubts about his fitness to be Autobot leader.
IDW comics continuity
In the ancient past of Cybertron's peacetime, Sentinel Prime operated out of the Security Services Headquarters in Kaon, generally regarded as "the worst place on Cybertron." Prime had much disdain for government authority, viewing the ruling Senators to be spoiled whiners. He worked alongside Prowl, his lieutenant.
After a surge in illegal gladiatorial bloodsport, he assigned Bumper and Fastback to stake out one of the sites associated with the matches as part of a dragnet. Unfortunately, their communications were jammed by Soundwave just prior to their untimely demises.
Sentinel Prime vowed not to let their deaths be in vain, and at their funeral, the Autobot leader urged others to let their deaths be a call to arms. Eventually, after Senator Decimus was kidnapped by the Seekers at Megatron's orders, Sentinel Prime led a raid on Megatron's base. They captured Megatron and most of the others, but were forced to free Soundwave per Senate directive, much to Prime's frustration.
When Starscream's "betrayal" led to the eventual eruption of the war, Sentinel Prime held the line at Kaon until he decided to take things to the next level. Thus, Prime returned with his massive "Apex" mode, a battle-armor/super-powerful/more-firepower-than-god mode. It didn't make much of a difference, seeing as Megatron tore into it and pulled Prime off a cliff. Sentinel was defeated by Megatron. Afterwards, the former Prime's barely functional body was dumped into the middle of the Autobot forces. Prowl ordered a retreat. We would too.
The next morning, Megatron discussed his next moves with Soundwave... sitting on a throne made out of Prime's Apex armor.
- As revealed in the Transformers Collectors' Club roundtable panel at BotCon 2007, the original concept for the convention-exclusive redeco of Cybertron Defense Hot Shot was Sentinel Prime. The mold DOES bear a decent resemblence to his IDW appearence, but in the end it was decided that Springer was a better choice for the mold (and likely for the ongoing club comic story).
- Nearly all of his technical specifications come from descriptions found in the Keepers Trilogy.