The American legend is helping Raonic through Queen's and Wimbledon and was watching on as world number two Murray recovered from a set down to beat his new charge in a gripping encounter.
A hard taskmaster, even Lendl must have been impressed by Murray's play after losing a first set tiebreaker and falling 3-0 down in the second set.
The Scot put together a winning run of eight games in nine to take command, breaking the big-serving Raonic at will with inspired play, eventually closing out a 6-7 6-4 6-3 victory at the grass court event.
True to his enigmatic nature, Lendl did not hang around to take the plaudits after Murray won match point, quickly vacating his seat.
It was not lost on Murray, who was also delighted to break out of a tie with eight four-time Queen's champions, including McEnroe and Novak Djokovic's coach Boris Becker.
"It's a pleasure to play in front of someone like John McEnroe and any time you can do something a bit better than someone like him, then that's amazing," he said as he looked in his direction.
"To win here for a fifth time means a lot for me. It was nice of Ivan to stick around for the presentation!"
Opponent Raonic, who showed he will be a dangerous contender at Wimbledon, wished Murray a happy father's day, his first after wife Kim, who was also at Queen's, gave birth to their daughter Sophia earlier this year.
Both finalists will now head to SW19, looking to topple Djokovic, who has won the opening two grand slams of the year, beating Murray in both finals.
Lendl was in Murray's camp when he achieved his two grand slam titles, beating Djokovic on both occasions, but split from him in 2014 citing the demands of traveling.
Meanwhile, the other major Wimbledon warm up tournament went to Florian Mayer, who beat fellow German Alexander Zverev 6-2 5-7 6-3 in Sunday's final.
Teenager Zverev beat eight-time winner Roger Federer in the semifinals in a major upset but could not recreate that form in the title clash as his opponent took a second ATP Tour title after injury problems.