Hamilton, who withdrew from his media commitments on Wednesday in the wake of Niki Lauda's death earlier this week, was six hundredths clear of Red Bull's Max Verstappen.
Hamilton's team-mate Valtteri Bottas finished third. The Ferrari duo of Charles Leclerc and Sebastian Vettel ended the opening running fourth and fifth respectively.
Mercedes are racing for the first time since Lauda, their non-executive chairman, died in Zurich on Monday night.
Their cars are adorned with the motif, 'Danke Niki', whilst team members are wearing black armbands.
Mercedes have struggled with the slow-speed nature of the Monte Carlo street Circuit in recent years – but Hamilton's early pace suggests they could be the team to beat.
The Silver Arrows head into Sunday's race having started the season with an unprecedented five one-two finishes.