The Canadian world number four was a doubt for this tournament after tearing a quadricep muscle in Paris last week, but he showed no signs of any issues as he eased to an ice-cool 6-3 6-4 victory.
Raonic, who reached his first major final at Wimbledon back in July, could yet finish the season in the top three with a strong week in London and on this evidence few players will relish the task of trying to neutralise his power.
It was always going to be an interesting match-up of styles with Monfils' penchant for the unpredictable and the spectacular contrasting with Raonic's power and serving efficiency. It was the latter that proved decisive in the first set with the Canadian staying consistent throughout a 34-minute exchange that saw him snare the only break in the second game.
Monfils, who was making his bow at this tournament at the age of 30, had produced the more memorable moments with a 'no-look' dropshot and 'slam dunk' smash, but he needed to call upon his fighting qualities at the start of the second set as Raonic pressed for the early break once again.
Gael Monfils in action during his round robin match with Canada's Milos Raonic
Image credit: Reuters
The Canadian was barely troubled on his own serve throughout and finally made the decisive breakthrough in the seventh game with a sliced crosscourt pass that Monfils misread.
Raonic then served it out on the second of three match points with his 12th ace of the contest to secure his first ever win at the O2 Arena and move him to the top of Ivan Lendl group ahead of Novak Djokovic, who beat Dominic Thiem in three sets in the afternoon session.