Neill died in Australia on Monday at the age of 78, his family announced earlier this week.
His family said the actor was cancer-free at the time of his death but did not initially disclose the cause.
"I spoke with his family and wish to clarify some details for his fans," longtime agent Philip Grenz said in a statement to Radio New Zealand.
"Sam passed away from pneumonia. Prior to becoming sick, Sam had valiantly fought and beaten lymphoma through a new treatment called CAR-T therapy."
Grenz said Neill s family would hold a private memorial service in New Zealand.
"As Sam was an intensely private man who loathed a fuss, his family will honour him with a private family memorial at his farm in New Zealand at a still-undetermined later date," he said.
The agent also revealed that Neill had completed work on four projects over the past year, all of which are expected to be released in the coming months.
Neill disclosed in his 2023 memoir that he had been diagnosed with stage-three non-Hodgkin lymphoma and was "possibly dying."
Earlier this year, however, he announced that he was cancer-free after undergoing CAR-T therapy, an advanced treatment that modifies the immune system to fight cancer.
Born in Northern Ireland in 1947, Neill moved to New Zealand s South Island as a child.
Originally named Nigel John Dermot Neill, he later adopted the name Sam, saying his birth name felt too "effete" for life in New Zealand.
He began his acting career in New Zealand films during the early 1970s before gaining recognition in Australia.
Neill achieved international fame in 1993 with his portrayal of Dr Alan Grant in Steven Spielberg s blockbuster "Jurassic Park," a role he reprised in later films in the franchise.
Away from acting, he owned and operated vineyards in the Central Otago region of New Zealand s South Island.
Following news of his death, tributes poured in from friends, colleagues, neighbours and Hollywood figures, including director Steven Spielberg and fellow "Jurassic Park" stars Laura Dern and Jeff Goldblum.
