The last male player to complete the double - winning Indian Wells followed by Miami in the same season - was Roger Federer in 2017.
But Sinner is the first male to do so without dropping a set in either Masters 1000 tournament.
"It's very, very special moment,'' Sinner said. "Coming here, performing in a good way after Indian Wells, means a lot to me. Physically, it's tough...You're a little bit tired, but the motivation is very high.
'The second-seeded Sinner didn't let either of the two 90-minute rain delays slow him on Sunday. He notched 10 aces in the final, and 70 in the tournament for the second-most of his career.
He often had world No.21 Lehecka running from side to side with his precise, deep groundstrokes.
Sinner, 24, won his first 23 first-service points — a streak that lasted midway into the second set.
He became the eighth male to run the table at Indian Wells and Miami, and he and Sabalenka became the first to accomplish the double in the same year since Novak Djokovic and Victoria Azarenka in 2016.
This was Sinner's second Miami Open title in three years as he ran his Hard Rock Stadium win streak to 12 matches dating to 2024. Last year, he missed the tournament serving a suspension for testing positive for a banned substance.
"It has been on my mind,'' Sinner said on Tennis Channel of missing last year's events. "I missed these type of tournaments. These are very important weeks.''
Lehecka, 24, in his first ATP 1000 final, was a big underdog, losing all three prior matches to Sinner.
