Leclerc was flawless off the line to hold the lead from front-row starter Oscar Piastri, but behind him chaos enveloped the field.
Carlos Sainz, starting third, tangled with Piastri and picked up a costly front-right puncture that forced him to park up at Casino Square, his race seemingly over on the spot.
Further down the field, however, a bigger incident was unfolding as Kevin Magnussen attempted to pick off Sergio Pérez in a battle at the back of the field.
Magnussen showed Pérez a wheel on the Mexican’s right-hand side on the run between the first turn, Sainte Devote, and Massenet at the top of the hill, but the Red Bull Racing driver was resolute on the racing line as it snakes up the twisting ascent.
Suddenly the gap between Pérez and the barrier vanished, squeezing Magnussen against the wall. The contact sent Pérez spinning out of control and Magnussen ploughing into him, with the pair making heavy contact with the steel barriers.
Pérez rebounded onto the track to collect Magnussen’s teammate, Nico Hülkenberg, to take all three out of the race on the spot.
All three drivers walked away unscathed despite severe damage to the Mexican’s Red Bull Racing car.
Further along, Esteban Ocon was engaging in friendly fire with Alpine teammate Pierre Gasly in a skirmish for 10th when a botched passing attempt at Portier saw him crunch the sister car against the wall. Ocon launched himself high into the air, terminally breaking his car, while Gasly was able to continue with heavy damage.
A 40-minute delay ensued to repair the barriers, and as a happy coincidence both Sainz and Gasly were able to retake the start with repairs.
But the smash effectively decided the race at the end of the first lap. With all drivers allowed to make their mandatory tyre changes during the red flag, the Monaco Grand Prix became a long 75-lap procession of tyre management to the chequered flag at the slowest possible speed.
‘DREAM’
But the dour nature of the race did nothing to dent Leclerc’s joy to have finally won the sport’s most famous race, having twice failed to convert pole position to victory at home in the last three years.
“No words can explain that,” he said. “It’s such a difficult race. I think the fact that twice I’ve been starting on pole position and couldn’t quite make it makes it even better in a way.
“It means a lot. It’s the race that made me dream of becoming a Formula 1 driver one day.”
Leclerc revealed he’d been thinking of his father in the final laps. Herve Leclerc died in 2017, before his son signed his first Formula 1 contract.
“I have to say that I was thinking of my dad a lot more than what I thought while driving,” he said emotionally. “Obviously he’s given everything for me to be here. It was a dream of ours for me to race here and to win, so it’s unbelievable.”
Leclerc was pushed gamely by Piastri, who turned a career-high second on the grid into his best-ever grand prix finish of second at the flag.
“Tricky race,” he said. “The pace at the beginning was incredibly slow, and I had one little half-look [at a pass on Leclerc] before the tunnel but didn’t have a small enough car to fit into a gap.
“Thanks to the whole team. It’s been a great weekend all round. It’s nice to finally put a result on the board. We’ve been very strong for the last few weekend but didn’t have the results to show for it.”
Sainz made the most of his lucky second chance to finish third, having pressured Leclerc in the final stages of the race as tyre life became most precarious.
“Obviously a very bad feeling there on lap 1 that very quickly turned into a good feeling after being reinstate in P3,” he said. “The race pace was good as expected, but it’s just impossible to get past on the streets of Monaco.”
Lando Norris completed an unchanged top four, the Briton cruising home without any meaningful threat behind him.
George Russell held off Max Verstappen in fifth and sixth ahead of Lewis Hamilton in seventh.
Yuki Tsunoda scored four points for RB in eighth ahead of Thai driver Alex Albon’s first score of the season for Williams in ninth and Pierre Gasly’s damaged Alpine in 10th to complete the point-scoring positions.
Fernando Alonso finished outside the points ahead of Daniel Ricciardo, Valtteri Bottas, Lance Stroll, Logan Sargeant and Zhou Guanyu.