During the spacewalk astronauts Patrick Forrester and Steven Swanson managed to activate a Solar Alpha Rotary Joint (SARJ), allowing solar arrays installed earlier to rotate towards the sun.
They also laid a computer cable and removed a Global Positioning System (GPS) antenna, as well as installing a defense shield on the Destiny laboratory to protect against space debris.
NASA chiefs decided to extend the current shuttle's mission by at least two days after an inspection revealed slight damage to its thermal blanket.
In 2003 similar heat shield damage led to the disintegration of a Columbia shuttle, which resulted in the death of seven astronauts on re-entry into Earth's atmosphere.
The ISS also experienced problems when the steering system on the Russian segment was disabled by computer failure, which experts believe was caused by changes in the ISS configuration after new solar arrays were connected.
Both problems were repaired during a third spacewalk.
A final test of the computer navigation system will be carried out Monday before the shuttle undocks Tuesday.
The Atlantis shuttle is due to return to Earth June 21.
