Finland's Valtteri Bottas was second, a hefty 20.5 seconds behind, with Dutch 20-year-old Max Verstappen completing the podium for Red Bull - his first of an incident-filled season. Ferrari's Sebastian Vettel could manage only fourth after his team took a second pitstop for the German behind the virtual safety auto that lost him track position.
The result sees Hamilton stretch his lead at the top of the drivers' standings to 17 points after five races.
Defending champion Hamilton has had to be patient this season, waiting until the fourth race in Azerbaijan to register his first victory of the campaign. He inherited victory at the last round in Baku but in Spain, having claimed pole with a superb lap, he converted it with clinical efficiency that exploited the performance he has sorely lacked from the Mercedes in three of the four previous rounds. This 74th career pole is a return to competitive form just when he needed it, having been outpaced over the single lap at Bahrain and China by his team-mate Bottas, and by Vettel in the last three races.
"This is more like it guys, let's keep this up", a buoyant Hamilton said.
The victory will be hugely pleasing and of some relief to the Briton.
The victory bodes well for Mercedes for the rest of the season as the performances in Barcelona - where most teams bring significant auto updates - usually set the tone for the rest of the races on the European portion of the calendar. This was also Mercedes' first one-two of the campaign and sent them back on top.
New Fissure Opens Downrift Of Leilani Estates
The new fissure is the 16th that has emerged since the beginning of Kilauea's latest eruption on May 3. She said the lava inside the volcano sank out of site, replaced with steam plumes rising into the sky.
Leclerc scored points for the second race in a row, setting him up nicely for the next in his home town of Monaco.
Hamilton led from pole position.
As he spun back across the track, Nico Hulkenberg tried to take avoiding action in his Renault but had his rear wheel caught by Grosjean's auto, and Toro Rosso's Pierre Gasly, unsighted, smashed into the side of the Haas.
All three drivers were unhurt but it was an exceptionally unsafe incident and why Grosjean kept his foot on the gas as he span, causing smoke that unsighted the following drivers, seems inexplicable. He was able to stay out a further eight laps, ultimately taking the medium rubber in a clean stop on lap 26.
And Wolff also revealed that the team, who eventually sealed their first 1-2 of 2018, were tempted to follow Ferrari onto a two-stop strategy.
With Hamilton once again in clean air out front he held the lead on Vettel at over ten-seconds, when the virtual safety vehicle was deployed after Esteban Ocon's retired at the side of the track on lap 41.
Ferrari pitted Vettel early from second, forcing Mercedes to react with Bottas a couple of laps later. He made two stops, while others made just one, and it cost him. He made the right strategy call by pitting when the virtual safety auto was deployed but ran over front wing debris from Verstappen's vehicle that caused some damage. Mercedes didn't pit either Hamilton or the now second-placed Bottas and both cruised to the flag on their one-stop strategy.