Mathieu upended No. 7 Sjeng Schalken of the Netherlands, 4-6, 6-2, 6-0, and Maleeva knocked off third-seeded Lindsay Davenport of the United States, 5-7, 6-3, 7-6 (7-4), in the finals of the $2.2 million indoor event.
The 20-year-old Mathieu became the first qualifier to triumph on the ATP this year. He upset top seed Marat Safin of Russia on Saturday to reach his first career semifinal.
"It's hard to realize that I won my first ATP title," said Mathieu, who won the final eight games of the match to finish it in one hour, 38 minutes. "It does not happen very often that a qualifier wins an event. I've been waiting for this moment for a long time. This is what you play tennis for."
After dropping the first set, the young Frenchman broke Schalken twice in the second and raced through the third in just 26 minutes to earn the $133,000 first prize. Mathieu won eight matches in nine days, losing the first set on six occasions.
"The first set was tough," he said. "I was nervous but then again I was always losing the first set here before winning so I knew I could come back. In the second and third sets, I played my best tennis of the week, even better than against Safin in the semifinals."
Until Sunday, Mathieu's biggest achievements were limited to juniors and the lower levels of professional tennis. He won the junior event at Roland Garros in 2000 and reached three finals on the Challenger circuit over the past two years.
Maleeva captured her third Kremlin Cup, but first since 1995. She notched her ninth career title and her second Tier I crown. Her first tournament victory since April 2001 at Budapest earned Maleeva $182,000.
"I consider Lindsay the best player in the world and beating her in the final, it's just unbelievable," Maleeva said. "She killed me a couple of times we played but I played very aggressively for the whole week here. I also played the big points very well, so I think I deserved to win."
The 27-year-old Bulgarian scored wins over world No. 2 Venus Williams, No. 7 Amelie Mauresmo and No. 8 Davenport this week and will move back into the top 20 at No. 16 when the rankings are released on Monday.
"I could care less if I'll be in the top 10 or not," she said. "I have never beaten three top 10 players before, so this definitely is the best week of my life."
Davenport's third loss to Maleeva in five career meetings means the 26-year-old American is still looking for her first title of the year. She returned from knee surgery in July and has made the semifinals in all six tournaments she has played.