Now, he finally steps out of his big brother's shadow for good.
Fidel's death late Friday at age 90 leaves Castro alone at the levers of power for the first time since health problems forced his brother to step down in 2006.
Raul Castro had served as his brother's defense minister ever since 1959, when Fidel took Havana by storm at the head of a guerrilla army.
Castro, whose steel-rimmed glasses hide a sharp gaze, was always the reserved little brother to the larger-than-life Fidel.
But as defense minister for nearly 50 years, he was also perfectly poised to impose himself as the island's strongman when Fidel faded.
After consolidating control, backed by the military, Castro began small steps toward opening up the sclerotic, Soviet-style command economy.
In parallel, he launched a series of diplomatic initiatives to forge new allies -- notably opening up to the old enemy the United States, where President Barack Obama's arrival in power in 2008 promised a chance for detente.
This shift had practical, as much as ideological, motivations: leftist ally Venezuela, whose oil largesse had kept Cuba going after the Soviet Union's collapse, was sliding off the economic rails.
Castro, 85, has patched up ties with the European Union and dramatically responded to the Obama administration's call for an end to their half-century standoff -- a rapprochement that saw the old foes restore diplomatic relations last year.
Cautious and with less flair than Fidel, Castro has patiently diversified Cuba's alliances, attracting foreign investment so that an economy still 80 percent under state control no longer relies so much on a single foreign backer.
Yet Castro has given little away of the Communist Party's monopoly on running internal affairs.
Raul cuts a distinctly different figure to charismatic Fidel, his narrow mustache standing in symbolic contrast to his brother's bushy black beard.
But historians say their partnership is what got Cuba through so many challenges -- be it more than half a century of a suffocating US embargo or the devastating collapse of all-important Soviet support at the end of the 1980s.
With Fidel now gone and Raul promising to step down in 2018, the regime now faces its ultimate challenge: ensuring that opening to European and US investors doesn't bury the revolution.
Paul Webster Hare, who teaches international relations at Boston University, said Castro was happy to have new US tourism dollars but unbending on the issue of allowing democratic freedoms.
"The blocks will be imposed on political grounds on the Cuban side. That would change the whole narrative of the Cuban Revolution, which Raul wants to preserve," said Hare, a former British ambassador to Cuba.
"Raul's main concern is not to endorse inequality like the Chinese have," he added. "He knows that the revolution would not survive long."
Born June 3, 1931 to a prosperous Spanish immigrant landowner and a Cuban mother of humble background, Castro grew up trailing behind Fidel and another older brother, Ramon.
He fought alongside Fidel to overthrow dictator Fulgencio Batista, then took over as head of the new military, the Revolutionary Armed Forces.
Under his leadership, the army pulled off a pair of audacious victories intervening in Cold War conflicts in Angola and Ethiopia in the 1970s and 80s.
After taking over, he did show a sense of humor -- Castro regime style -- when in 2013 he told journalists he was resigning.
Friends say he is easygoing and amiable in private, despite his imposing public persona.
Castro met his late wife, Vilma Espin, when both were young rebels.
He married her in 1959, the year the revolution triumphed. She died in 2007.
He has four children.