Hallie Mossett was born on March 15, 1995. She trains at All Olympia Gymnastics Center in Los Angelas, California. Hallie started gymnastics classes at very young age, since then she has worked her way up to compete at the highest level of gymnastics. She made the National TOPs Team in 2003, 2004, 2005 and 2006. Now, Hallie is a junior international elite coached by Galina Marinova and Arthur Akopyan.

As a level 5 in 2003, Hallie won a several meets. She swept the gold medals at Sectionals. At Level 5 States, she won gold medals on every event except for vault. Hallie moved up to level six in 2004. She won the Crystal Cup and AOGC Invitational. At Sectionals, she swept the gold medals. Hallie took second in the all-around at Level 6 States. She also won vault and floor exercise. A few weeks later, she competed as a level seven. She finished second in the all-around at the Lady Luck Invitational. Her last level seven meet was the San Diego Classic which she won.

Hallie finished the 2005 season as a level eight. She placed third in the all-around at the Level 8 State Championships. At Level 8 Regionals, she placed fourth in the all-around. In 2006, Hallie moved up to level nine. She finished first in the all-around at the WOGA Classic, Go For It Classic and AOGC Invitational. She also won the State AA title, and skipped Westerns to work on pre-elite training. In 2006 Hallie became the first ever HOPEs National AA champion.

When 2007 season came by, Hallie competed as a level 10 and international elite. As a level 10, she placed second in the all-around at the WOGA Classic and won floor exercise at Level 10 State's. Hallie chose not to compete at Level 10 Regionals or Junior Olympic Nationals. Instead, she focused on her dream, elite. In May, Hallie went to National Training Center for the National Elite Qualifier. She easily passed the junior international level and she also placed first in the all-around, both in compulsory and optionals. Next was US Classic in Battle Creek Michigan. She placed 10th in the all-around and was the floor exercise champion. Hallie also qualified to her first VISA Championships in San Jose, California where she placed 16th in the all-around.