Sandeson was accused of killing fellow Dalhousie University student Taylor Samson
The jury, which deliberated for about 22 hours, returned its verdict at noon on Sunday.
First-degree murder carries an automatic life sentence without eligibility for parole for 25 years.
Sandeson, 24, was on trial for the first-degree murder of Taylor Samson, a fellow Dalhousie University student.
As Sandeson was led out of the courtroom after the jury's verdict was announced, Samson's mom said to him, "Turn around and take a bow, Billy."
The jury began its deliberations on Thursday afternoon after Nova Scotia Supreme Court Justice Josh Arnold spent 3½ hours instructing jury members.
During the trial, court heard that both Sandeson and Samson were involved in the drug trade.
The prosecution alleged Sandeson was in money trouble and set up a drug deal with Samson on Aug. 15, 2015. When Samson arrived at Sandeson's Halifax apartment, the Crown says Sandeson shot him to death and took his nine kilograms of marijuana. Samson's body has never been found.
The defence argued during its closing arguments that Sandeson is not a criminal mastermind and urged the jury to find him not guilty.
During court testimony, neighbours who lived across the hall from Sandeson testified that on Aug. 15, 2015, they heard a loud bang and saw a man with dark, curly hair sitting at Sandeson's kitchen table, bleeding profusely. They also saw drugs and cash.
They left, and when they returned later, they saw bloody streak marks on the floor leading toward the bathroom.
A shower curtain was later found on the Sandeson family farm with Samson's DNA on it.
In the days after Samson disappeared, Sandeson's brother Adam found a large quantity of marijuana in his basement after William Sandeson told him he was dropping by to do some laundry. Adam Sandeson and his roommates contacted a lawyer and the drugs were handed over to the police.