He destroyed the passport at the airport, the court heard.
While Marong had been living and working in New Zealand legally for a period, at the time of Duckmanton's murder he had been in the country illegally. He had been issued with a deportation notice, it's been confirmed.
Marong was born and raised in Gambia's capital city of Benjul as a member of the Mandinka tribe.
After secondary school, he studied computer science and commerce at university before dropping out to drive a taxi.
He left Gambia on August 28, 2012, leaving behind a wife and three children.
After going to neighbouring Senegal, he went to Hong Kong for 14 months before landing in New Zealand.
Further details of his time in New Zealand are suppressed.
He will be sentenced for Duckmanton's murder on April 20.
The jury dismissed Marong's claims that he had been "disconnected from reality" at the time he killed Duckmanton, and that from May 6-17, he was poisoned by kidney failure that resulted in urine "travelling in my brain".
A psychiatrist and a psychologist, who both assessed Marong before he stood trial, concluded that he did not meet the criteria for a defence of insanity.
Forensic clinical psychologist Ghazi Metoui found that Marong reported a settled mental state over the period "leading up to, during and subsequent" to the killing.
Dr Erik Monasterio, a forensic psychiatrist, found Marong's claims of insanity to be "farcical" and "preposterous".
After the guilty verdict, Detective Inspector Darryl Sweeney said it had been a "harrowing case for everyone involved".
"The result won't provide closure or solace for Renee's family, but it is important for everyone that they have answers and that the offender has been held to account," he said.
