In 2014, during the most prestigious freediving competition in the world, Vertical Blue, which takes place in the Bahamas, Marianna Gillespie imposed herself on the international scene, reaching, in constant weight with monofin, a depth of -86 meters, thus winning a silver medal. During the same tournament, she won two more medals: gold in free immersion and silver in constant weight without fins. She became the youngest athlete to win Vertical Blue overall.
During the same year, taking part in the Team World Championship, competing in one team with Natalia Molchanova and Marina Kazankova, she won her first World Champion title.
In 2015, her results ensured her, once again, the rank of World No. 1, all disciplines combined.
In 2016, Marianna decided to continue her progression in depth, focusing on the discipline of constant weight with monofin and reached -93 meters, thus taking 4th place in the world in the discipline.
That same year, she also brought a bronze medal to her team at the Team World Championship.
After a year break, in 2018 Marianna returns to depth competitions winning Sabang International Freediving Competition not only overall, but also receiving gold medals in all individual depth disciplines.
In 2019, at the World Championship in Villefranche-sur-Mer, Marianna competed in all individual events and won the gold medal in free immersion and two silver medals in constant weight with monofin and without fins. These are her first individual titles at the World Championships. She is titled best athlete of the World Championship and takes the absolute position of World No. 1 in the depth category.