The 20-year-old Russian goes by the nickname of 'The Bengal Tiger' but rather than knocking over her rivals with brute force, she simply left them chasing shadows as she produced four dazzling routines with the hoop, ball, clubs and ribbon to win her first global all around title.
To do that she beat overwhelming favourite and three-time world champion Yana Kudryavtseva, who surrendered her title hopes when her twirling act with the clubs went horribly wrong in its dying seconds.
"It was quite unexpected for me to win the gold medal today because before today Yana beat me and win each time in the all around. Hence I wasn't really thinking about winning the gold medal today," said the 20-year-old Mamun, who earned Russia a fifth successive gold in the discipline.
That error knocked a distraught Kudryavtseva, who was leading at the halfway point of the competition, off the top and allowed Mamun to grab the top prize with a total of 76.483.
So had Mamun instantly been aware of Kudryavtseva's mistake?
"I wasn't aware of Yana's mistake because I was backstage changing costumes for my next ribbon routine," she said after relegating her Russian teammate to the silver.
"I only saw Yana's score when I and Yana had both finished performing with the ribbon (about 30 minutes later and her score flashed up).
"I was really surprised Yana made the mistake as she is usually so focused and never shows any nerves."
"I'm really happy knowing that I have a lot of fans in Bangladesh who have been supporting me," added Mamun in Russian.
"I can count one to 10 in Bengali. When I was younger my dad used to teach me Bengali but I have forgotten it all."
And why did she chose to represent Bangladesh as a junior considering she was born and raised in her mother's homeland?
"I had dual citizenship so that's why I decided to represent Bangladesh in one competition as a junior. I came back to represent Russia as I always lived and trained in Russia."
Bangladesh have never won an Olympic medal.