Sinner had to wait out rain delays before and during the match to complete his "Sunshine Double," becoming the first man to sweep the elite hard court tournaments in California and Florida since Roger Federer in 2017 and the first to do so without dropping a set.
In fact, Sinner - who won Miami in 2024 but missed last year as he served a three-month doping ban - has now won three straight Masters 1000 events, starting with a victory in Paris last year.
"It's a very, very special moment," Sinner said. "Coming here, performing again in a very good way after Indian Wells means a lot to me.
"It's something I never would've thought (I'd win) because it's difficult to achieve."
His victory, coupled with Aryna Sabalenka's triumph over Coco Gauff on Saturday, marks the first time the Indian Wells-Miami sweep was achieved on both the men's and women's sides since Novak Djokovic and Victoria Azarenka both did it in 2016.
Early showers saw the match delayed for more than an hour. Sinner had just pocketed the first set when the showers hit again, halting the match for 90 minutes.
It wasn't enough to unsettle Sinner, who handed 22nd-ranked Czech Lehecka his first service break of the tournament for a 2-1 lead in the opening frame.
Sinner trailed 0-40 in the following game but fired three service winners and a pair of aces to hold and made the advantage stand up.
"I tried to be focused," Sinner said. "I tried to understand why I missed the first serves before. In the beginning, the conditions were very different ... very, very heavy balls. I missed a couple of first serves in the net because they were quite heavy."
He said Lehecka's aggressive return game also gave him pause.
"You have to hit very precise. If not, he has it on the racquet," Sinner said.
The Italian gave himself a first set point in the ninth game with a blistering cross-court forehand service return winner.
Lehecka saved that one with a service winner and saved another set point with a volley, going on to seal the hold on his sixth game point before Sinner pocketed the set with a love game.
Lehecka fought off five set points before Sinner pounded for the lone break of the second set and a 5-4 lead. He gave himself a match point with a forehand volley winner, thought he had it on a serve that was called a let, and polished it off with another winner at the net.
While Lehecka, playing in his first Masters 1000 final, couldn't follow compatriot Jakub Mensik into the Miami winner's circle, he will reach a career-high 14th in the world rankings on Monday.
"I came here with not good form and I was able to come back to the tennis that I want to play," Lehecka said at the trophy ceremony.