The pair replaced six gyroscopes that help point the telescope with precision at stars and galaxies.
Watch "World News with Charles Gibson" Tonight For More on the Hubble Mission
It was not easy. The gyroscopes were mounted deep inside the telescope, and Massimino, in his bulky pressure suit, had to wiggle into place to pull the old, failing gyros out.
The gyros are so accurate they can keep Hubble focused on a penny 200 miles away. But three of the six on board had broken down completely, and two others were misbehaving. Only one still worked properly.
The mission to repair and update the telescope so it can continue to help "unlock the secrets of the universe," has been years in the making.
The "secrets" phrase has become the lighthearted mantra for seven astronauts on the space shuttle Atlantis who are ssigned to repair the ailing orbiting observatory.
Since "Mike and Mike" can get confusing in radio communications with the ground, nicknames inevitably take over. Massimino becomes "Mass;" Good takes on the Spanish "Bueno."
How does an astronaut maneuver in the tight innards of the world's most celebrated telescope -- especially when he's wearing a backpack and a helmet with limited visibility?
"You know you're out of room," Massimino told ABC News before the flight, "when you hit something."
The astronauts took an hour an a half longer to replace the gyroscopes than had been planned. Mission Control warned them they would run short on oxygen and batteries if they continued the spacewalk too long.
But by afternoon they said they were in good shape, closing the large doors at the base of the telescope.
"Mass and Bueno, my friend (King) Leonidas has a couple of words for you guys that are appropriate right now," Atlantis commander Scott Altman joked. "'Remember this day, men, for it will be yours for all time.'"
"Thanks, Scooter. We've got a little more work to do, but thanks," Massimino replied. "Thank very much."
Astronauts Massimino and Good floated out of Atlantis' airlock shortly before 9:00 this morning, EDT.
Massimino has made two spacewalks before. He was on the crew of the last shuttle mission to the Hubble in 2002. Good is a rookie.
"Welcome to the wonderful world of working in a vacuum," called crewmate John Grunsfeld from inside the cabin as Good made his first foray into the near-vacuum of space, 350 miles above the Earth's surface.
Good's job today was to install parts while perched on the end of the shuttle's robot arm. Massimino was assigned to play contortionist, climbing inside the telescope on a thin tether.
Massimino is the lead spacewalker. He is a gregarious New Yorker, an avid Mets fan who brought a home plate from Shea Stadium along on the flight.
Mike Good helped Massimino with tools and positioning. Before the flight he talked with ABC News about standing in his driveway one night in Houston, watching the tiny, starlike Hubble streak across the night sky, and wondering what it would be like when he was fixing Hubble.