Mar-Vell was a member of the blue-skinned Kree empire. At birth he was as pink as a human. The only other abnormality was the Star Brand on his right hand. To some, he was the fullfillment of Kree prophecy. To others, he was an abomination, but despite discrimination and prejudice, Mar-Vell became a renowned soldier and an acclaimed hero for his victories in battle against the Kree's enemies, notably the Skrulls.

Mar-Vell's regiment, under the command of Yon-Rogg, was given a mission to observe Earth, specifically human's progress of space travel. Yon-Rogg, jealous of the love between Mar-Vell and the ship's medic, Una, ordered Mar-Vell to infiltrate Cape Canaveral, Florida, alone. He then secretly tried to fire on Mar-Vell's ship, but killed a human instead-- Ken Connell. Mar-Vell took advantage of resembling Connell to pose as a human and create a secret identity, but Yon-Rogg again secretly tried to kill Mar-Vell by activating the Kree robot, the Sentry. In his Kree uniform, Mar-Vell publicly defeated the robot, and the onlookers acclaimed him as "Captain Marvel," misinterpreting what the Sentry called him.

[Editor's Note: It is here that Sebastian Shaw acquired the Sentry robot.]

Mar Vell continued to observe Earth even though Yon-Rogg returned home. Knowing  that Rogg would be back, with the Kree Armada, Mar Vell set out to join together the greatest group of heroes the world has ever known: Ultraforce!

He first sought out the media darling of the day, Tony Stark's creation, the Vision. Mar Vell admitted he knew that Stark was actually in the armor, and promised to show him technology undreamt of by mortal minds. Stark agreed to join.
Next he met Thor. Thor challenged him to the Right of Combat, where Thor ended up winning, but Mar Vell accquitted himself well. It seems Thor had been sent to the realm to become it's defender, he was happy to join.
At the behest of Stark, Mar Vell approached Hank Pym and Janet Van Dyne, a married couple woking in his company, Force Works. They had been developing a super team to back up the Vision and were ready to try some of the concepts out.
Lastly, they approached the king of the soveriegn nation of Wakanda, T'Challa: The Black Panther. Not only his nation's hero, but also a man of great influence and importance.
On their first outing to engage the Enchantress' Wrecking Crew; the Force quickly dispatched of the quartet and was met by S.H.I.E.L.D. After long talks behind closed doors, Nick Fury allowed them to operate autonomously under the condition that SHIELD could appoint three members to their group. Mar Vell and Stark agreed and so the Force added:

Captain America: the Sentinel of Liberty.

Black Widow: International Agent of Espionage.

The Paladin: Weapons Expert Unsurpassed.

The Ultraforce were housed at Force Works and kept close ties with SHIELD.

Until The Kree Invasion happened.

After the Invasion Captain America quit SHIELD and Ultraforce. Stark quit and booted them out of his compound (with the exception of Pym and Van Dyne who stayed with Stark). Thor returned to Asgard in disgust having failed in his duty. Mar Vell sat aboard the SHIELD helicarrier and expressed his remorse to Fury. As he was about to leave, Fury stopped him.
"Mar Vell, you are a captain, a soldier....what if we rebuild Ultraforce under your leadership? You can look at our prospects and choose your team. Would that convince you to stay, as a SHIELD extension?"

"Yes......but we will need a veritable army. The world is now on the cosmic radar, and I know the evils that lie out there. Show me your prospects."

After about 4 hours of deliberation, Fury and Mar Vell assembled the new Ultraforce:

Captain Marvel, the leader.
Black Widow, 2nd in command.
Paladin, Instructor.
Black Panther, King.
Doc Samson, survivor of the HULK virus.
Blayze, Lt. Monica Rambeau.
Moondragon, Cassandra Nova the twin sister of Charles Xavier.
Thundra, Princess Zarda from another dimension.
Songbird, Melissa Gold reformed Master of Evil.
Tigra, former cat burgular Felicia Hardy.
Spitfire, Jenny Nai Swenson half sister of Tony Stark.
Justice, John Tensen former detective now Cosmic Cop.
Ant ManII, Scott Lang Hank Py's apprentice.
Mantra, Maiden of Mysticism.
Prime, 13 yr old Kevin Green is the World's Mightiest Man.
Together, they are the Earth's Mightiest Heroes: ULTRAFORCE!