how did Sarge get off halo?

Answer 1

According to the novels, prolonged exposure to plasma grenades created a physical condition that protects him from the flood.

As for how he got off.. the ring exploded, but it didn't wipe-out all lifeforms on it.

Supposedly the graphic novel shows his escape, but I have it, and it doesn't. It shows him getting out to the opening of the building they were in when Jenkins died. (remember? the guy's helmet where you watch the recording of them fighting flood?)

It also shows a conversation that suggests Johnson may be part of the Spartan One program, and/or part of OMI's Section 3. In theory, that makes him important enough to get picked up fast, and would possibly require him to keep the details classified.

However, there's still no official explanation on how he phyiscally managed to get off the ring. Especially if you saw the Halo 1 legendery cutscene at the end of the game that showed him hugging an elite with stuff blowing up in the background behind them!

**Update**
Actually, according to the Halo novel: "First Strike":

Johnson managed to get to a Pelican with a pilot named Polanski as the Ring was blowing apart. At the end of Halo 1, when Cortana says there's nothing there but "dust and echoes", she was doing a passive scan (which means listening for transmissions) instead of an active one. She didn't want to do an active scan because that means looking at billions of chunks of debris and trying to identify each piece of it. Spartan-117 insisted, and as soon as she did an active one, they found the pelican beginning a series of evasive maneuvars as enemy ships cane into the area.

They "docked" with it (kinda.. not quite, but close enough) and retrieved Johnson from it, before dealing with the incoming covenant ships.

(The novels are great, btw,.. totally worth reading)

Answer 2

If you mean Halo 1, he must have stolen a ship like Master Chief, but In Halo 2, when asked how he escaped, Sarge said it's classified. Also he has a brain disorder that could have protected him from the Flood. I believe Master Chief is the only one who knows how Sarge escaped