He appeared at a gala in Havana's Karl Marx Theatre with his brother, President Raul Castro, and Cuba's ally, Venezuelan President Nicolas Maduro.
In his first public appearance since April, he appeared frail and remained seated during the event.
Fidel Castro, who stood down in 2008, had earlier attacked US President Barack Obama in a newspaper column.
He criticised Mr Obama for not apologising to the people of Hiroshima for the nuclear bomb dropped there by the US in World War Two. Mr Obama visited the Japanese city in May.
"He lacked the words to ask for forgiveness for the killings of hundreds of thousands of people," Mr Castro wrote.
Ninety years of Castro and the Queen
Fidel, the great survivor
Ties between the US and Cuba have been restored under Raul Castro's presidency, but after a visit by Mr Obama to Havana in March, Fidel Castro wrote that "we don't need the empire to give us anything".
The gala in Havana focused on key moments of Fidel's life, including the CIA-backed invasion attempt in the Bay of Pigs in 1961.
A large street party was also held in the capital late on Friday, and fireworks exploded when the clock hit midnight.
The BBC's Will Grant in Havana says that Fidel Castro now has all but retired from public life and is thought to exercise limited influence over political affairs in Cuba.
The former Cuban leader Fidel Castro has made a rare public appearance at an event to mark his 90th birthday.