Maldonado's win, in just his 24th race, was also the team's first victory since the 2004 Brazilian Grand Prix and came on the weekend of team founder Sir Frank Williams' 70th birthday.
But on a dramatic day for the Venezuelan driver, he was later pictured on Twitter carrying his young cousin to safety after the team garage was engulfed by flames.
A total of 16 people were injured, one with severe burns, when the fire -- believed to have been caused by an electrical fault in a fuel rig -- broke out.
Maldonado was forced to carry his 12-year-old cousin Manuel, who was wearing a protective boot on a broken right foot, to safety as thick, black smoke billowed down the pit lane at the Catalunya Circuit.
"After today's Spanish Grand Prix a fire occurred in the team's garage which originated from the fuel area," said a Williams team statement.
"Four team personnel were injured in the incident and subsequently taken to the medical centre. Three are now receiving treatment at local hospitals for their injuries, while the fourth has been released.
"The team will monitor their condition and ensure they receive the best possible care. The team, the fire services and the police are working together to determine the root cause of the fire and an update statement will be released in due course."
The Catalan regional government said that one person was airlifted to hospital with serious burns, while five others were taken to a local hospital for the effects of smoke inhalation.
Ten more were treated at the circuit's medical centre.
The Caterham team said four of their mechanics were among those taken to the medical centre, one with a minor hand injury and three for smoke inhalation.
On the track, Maldonado held off a challenge by two-time champion Fernando Alonso of Ferrari to win.
"It's a wonderful day for me and the team. An unforgettable race," said Maldonado, who was hoisted onto the shoulders of Alonso and Kimi Raikkonen, who finished third, when the drivers appeared on the podium.
Driving with huge self-control, the Venezuelan produced a flawless race for the Williams team, delivering their first win since Colombian Juan Pablo Montoya won in Brazil eight years ago.
Maldonado had also become the first driver from his country to take a pole position when he was elevated from second place on the grid on Saturday evening, following Lewis Hamilton's disqualification for fuel irregularities.
Maldonado drove with great composure in the closing stages to make the most of his team's clever strategy by holding off two-time champion and local hero Alonso.
"After last year, we have worked and worked, and pushed so hard, and I knew this was going to be a tough race, but to have my first podium and my first win -- you can just imagine the emotions," said the winner.
Alonso said: "It was a fantastic start and it was close into turn one, but we had the better side and we had the momentum to lead for the first part.
"But Williams anticipated the first stop better and as the race went on, we lost grip and maybe some aero and in the end I was in the battle with Kimi for third."
Raikkonen, who was third for Lotus, said: "I expected to be a bit stronger in the race, especially at the beginning. At end of the race, we were good, but it was too late."
Romain Grosjean came home fourth in the second Lotus ahead of fifth-place Kamui Kobayashi of Sauber and defending double world champion Sebastian Vettel of Red Bull.
A late charge by Vettel enabled him to rise to sixth by passing fellow German Nico Rosberg, who was seventh for Mercedes, and Hamilton, who also drove superbly to rise from 24th and last on the grid to finish eighth.
Hamilton's McLaren team-mate and compatriot Jenson Button came home ninth and Nico Hulkenberg was 10th for Force India on a day when Maldonado became the fifth different winner in five races so far this year; the first time this has happened since 1983.
Vettel's late push enabled him to maintain his position at the top of the drivers' standings, where he shares the lead, on 61 points, with Alonso, eight points ahead of Hamilton.
In the constructors' championship, Red Bull stayed out in front with 109 points, ahead of McLaren on 98.