But the Ferrari driver's commanding victory was overshadowed by Alonso's opening-corner smash in which the Spaniard was sent airborne and within inches of Sauber driver Leclerc's head.
Images of Leclerc's white halo, the three-pronged cockpit protection device which sits above the driver and became mandatory this season - later emerged to reveal the black marks of Alonso's McLaren.
The dramatic collision was caused by Nico Hulkenberg after the experienced Renault driver missed his braking point at La Source and, in a plume of tyre smoke, careered into the back of Alonso.
The force of the impact launched the double world champion skywards before his McLaren landed on the helpless Leclerc, and then on the tarmac. Miraculously, however, all three men walked away completely unscathed.
"I flew over Leclerc's car and the halo was a very good thing to have today," Alonso said. "Looking at the replay I think it helped him.
"We didn't have any doubts about it before, but this accident shows it offers good protection."
Team principal Toto Wolff, who revealed at Mercedes' car launch in February that he wanted to take a chainsaw to the halo, said: "As you know, I am not a fan of it because I think the aesthetics are terrible.
"But having saved Charles from harm and injury it makes it all worth it. It could have been very nasty."
Hulkenberg admitted fault for the incident and was given a 10-place grid penalty for the next race in Italy. He was also hit with three points on his drivers' licence.
"Nico totally missed the braking point and he played bowling with us," Alonso added. "The drivers think it is the last corner of the championship and it has high consequences. You think they would be at a higher level to drive in this series."
The halo was controversially introduced following a series of high-profile accidents in the past decade.
Leclerc, the highly-rated 20-year-old who has impressed in his rookie season and is set to race for Ferrari in the future, revealed his mother, Pascale, called him several times after the incident.
"Everyone was quite worried," the Monegasque said. "My physio sent a picture to my mum so she knew I was fine. She was then calm when I later spoke to her on the phone. She asked: 'Are you OK?' Like any mother, I guess."
Regarding the incident, he added: "It all went very quickly and in the moment you don't realise what is happening.
"I felt the impact and looking at the image of my car it is quite spectacular. I was lucky."
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