Mbule joined the Buccaneers in July last year on a season-long contract that included an option for the club to extend his stay.
However, after making 23 appearances across all competitions, Pirates opted against activating the clause before the May 31 deadline, effectively making the midfielder a free agent.
The decision does not come as a major surprise given Mbule's reduced involvement during the second half of the season.
After returning from the Africa Cup of Nations, the 28-year-old slipped down the pecking order and featured only sporadically as Pirates went on to clinch the Betway Premiership title from Mamelodi Sundowns.
Speaking to the public broadcaster, ProSport International representative Mike Makaab confirmed Mbule's departure and explained that clubs are not obligated to provide reasons when deciding against exercising an option clause. According to Makaab, Pirates simply chose not to trigger the extension before the deadline.
Mbule was handed a career lifeline by the Soweto giants after his departure from Sundowns, where his spell ended amid controversy.
His loan stint at Sekhukhune United had also concluded unexpectedly despite him playing a significant role for the club.
There were signs of a revival during the first half of the campaign, with Mbule forcing his way back into the Bafana Bafana setup and appearing to strengthen his chances of featuring at the FIFA World Cup.
However, a decline in form after AFCON ultimately saw him miss out on Hugo Broos' final squad, and now, his time at Orlando Pirates has also come to an end.