He's not only the retired leader's personal photographer, he's also a son of the former Cuban president.
The younger Castro was known for his pictures of Haiti and ballet in Cuba, but he has spent much of the last decade since his father fell ill and stepped down from power documenting those twilight years.
"There are times when he wants his privacy," Alex Castro said of his 90-year-old father in an exclusive interview with CNN. "I take the majority of these photos when there is a visit, when he is working. But the private times I don't do anything. It's private."
During the 49 years he ruled Cuba, Fidel Castro insisted on privacy concerning all matters of his personal life. Cuba's state-run press never even mentioned his family, though many Cubans were aware he had a son from a first marriage, other children out of wedlock and was married to a woman named Dalia Soto del Valle with whom he had five sons.
But after a 2006 intestinal illness forced Castro from power, more details began to emerge about his family.
As rumors swirled over Castro's health, Alex Castro's photographs of his father meeting with Russian President Vladimir Putin, US movie director Oliver Stone and Argentine soccer star Diego Maradona functioned as a sort of proof of life.
"They are not so much official visits as visits from friends and admirers," Alex Castro said of the string of meetings that along with the writing of opinion pieces occupy his father's life.
In 2012, Fidel Castro and his family were photographed meeting with Pope Benedict XVI during his trip to Cuba, the first images that many Cubans had ever seen of Castro with his children.
In 2015, Alex Castro published a book called "Fidel Castro: An Intimate Portrait," and in August he participated in a photo exhibition in Havana to celebrate his father's 90th birthday.
His photographs show the former Cuban president meeting with politicians, actors and activists at Punto Cero, or Zero Point -- his heavily guarded home in Havana surrounded by vegetable gardens he oversees. These bucolic years were probably not the end the Cuban leader expected after decades of challenging the United States, imposing a Soviet-style centralized economy on the Caribbean island, stifling internal dissent and predicting he would die while still in power
Whether his father is meeting former US President Jimmy Carter or Pope Francis, Alex Castro says there is one thing he never does while capturing the moment.
"I am outside," he said. "I take the photo, and I don't listen to what they talk about. I prefer not to listen."
The photos are among the remaining few windows into the life of the controversial ex-Cuban leader, once famous for giving hours-long speeches haranguing US policies and letting American media outlets tag along as he traveled by military jeep and helicopter across the island.
The photos capture him as he shows Chinese President Xi Jinping his vegetable garden, shares a joke with French President François Hollande and signs a baseball for Carter.
On Monday, the son photographed his father meeting with Iranian President Hassan Rouhani as he sat in front of a stained glass window in the family living room. On Thursday, it was Japanese Prime Minister Shinzo Abe's turn to chat with Castro. The men discussed the need to abolish nuclear weapons, among other topics, according to Cuban state-run media.
There are few photos, however, of Fidel's younger brother and successor, President Raul Castro.
"I have few photos of Raul because what I do is take photos of my father," Alex Castro said, before adding that his uncle visits the house often.
One photo shows his famous father standing over a life-size statue of himself sculpted by a Chinese artist.
"He receives many gifts, but you realize in Cuba there aren't statues of him.
"He doesn't permit it, " the younger Castro said, insisting his father doesn't care for the trappings of power and continued worldwide interest in him.
"So they give him these gifts, but they are stored away," he said. "He doesn't like the cult of personality. He is a simple person and doesn't want to be worshipped."