Dhak is believed to have been a prominent local gang leader, while Mantel has been described as his bodyguard.
The two were gunned down in the lobby of the Executive Hotel and Conference Centre on the Lougheed Highway just before noon on Monday.
A witness told CBC News a man walked up to the hotel's glass doors and opened fire.
No suspects have been arrested in the shooting and police have released few other details about the case.
Last year, the police issued a warning that anyone associated with Sukh Dhak was at risk. More recently, the Crown had outlined a case against Dhak, calling him the mastermind behind a large-scale drug lab.
Dhak's brother, Gurmit Dhak, was shot and killed outside the Metrotown mall in Burnaby in October 2010.