But Nour has always maintained the charges were politically motivated and designed to undermine his political rise.
He accused the Egyptian government of punishing him for daring to challenge Mubarak, who has ruled the most populous Arab country since 1981.
Egyptian authorities said they decided to release Nour, who is diabetic and has had heart problems, as his health was deteriorating.
His health conditions require extra care that cannot be provided behind bars, Gameela Ismail, his wife, said.
Nour told the Associated Press news agency from his Cairo home that he had not been notified of his release until a car came to pick him up at his prison and brought him home.
"Why they did this is unknown ... I am coming out with an open heart and am ready to work, and nothing has changed. A lot of things have been put on hold over the past years," he said.
Nour said that he planned to continue his work in politics through the opposition al-Ghad party, though Egyptian law prohibits him from seeking public office barring a presidential pardon.
"Thanks to God I am released," he said. "I am going to practice my role as a politician through the al-Ghad party."
Nour's imprisonment had been a sticking point in Egyptian-US relations for more than three years, and his sudden release may be interpreted as a concession aimed at improving ties with new administration of Barack Obama, the US president.
In August, Nour wrote a letter to then-presidential candidate Obama, urging him to help Arab reformers push for democracy in the Middle East.
In the letter, Nour said Obama "embodies the dreams of Arab reformers for democracy and change".