Mostert attended New Smyrna Beach High School in New Smyrna Beach, Florida , where he played football and competed in track and field . Mostert led New Smyrna Beach to two playoff berths. After his senior season, he moved on to Purdue University .
In his first collegiate season, he set a school record as he averaged 33.5 yards a return,  capped off with a 99-yard touchdown return in the bowl game. He finished the season with seven returns of 39 or more yards, including an 81-yarder at Indiana and a 74-yarder at Wisconsin . Against the Badgers, he racked up 206 yards on five kickoff returns to break 42-year-old school records for total yardage and average yards per return. 
High school career
Mostert attended New Smyrna Beach High School , in New Smyrna Beach, Florida , where he was a member of the football and track & field teams.  As a football player under coach was Lance Jenkins, he returned nine kickoffs and one punt for touchdowns, in addition to having 39 receptions for 723 yards (18.5 average) and four touchdowns as a senior.  He also recorded 81 tackles on defense. As a participant in the 2010 Central Florida All-Star Game,  he was named MVP after hauling in three passes for nearly 100 yards and returning a kickoff 94 yards for a touchdown.   He also made an appearance in the classical Florida North Vs. South football game.
Track & field
As a track athlete under coach Brendan Robinson, Mostert was a standout sprinter, hurdler and jumper. He won the 2010 FHSAA 3A District 6 title in the 300-meter hurdles.  At the 2011 FHSAA 3A Outdoor State Finals, he finished first in the 100 meters , with a time of 10.68 seconds, and placed fourth in the 300-meter hurdles , with a personal-best time of 37.95 seconds. 
He was a member of the Purdue track team. He was ranked by NFL.com as the fastest college football player in the NCAA , with only Oklahoma State wide receiver Tyreek Hill possessing a faster 100m time (9.98 s). He took gold in both the 60 meters (6.63s) and 200 meters (20.73s) at the 2014 Big Ten Indoor Track & Field Championships.  He qualified for the 2014 NCAA Outdoor Track and Field Championships in the 100 meters (10.15s), 200 meters (20.65s) and as a member of 4x100-meter relay squad. 
Mostert committed to Purdue University on January 14, 2011. He chose Purdue over football scholarships from Illinois University , Indiana University , Marshall University , the United States Naval Academy , Rutgers University , the University of Miami , the University of Southern Mississippi , the University of Central Florida and the Wake Forest University . 
|Name||Hometown||High school / college||Height||Weight||40 ‡||Commit date|
|New Smyrna Beach, Florida||New Smyrna Beach High School||5 ft 11 in (1.80 m)||183 lb (83 kg)||–||Jan 14, 2011|
|Recruiting star ratings : Scout : Rivals : 247Sports : N/A|
|Overall recruiting rankings: Scout : – ( S ) Rivals : – (WR), – ( FL )|
In his first collegiate season in 2011, he set a school record as he averaged 33.5 yards a return,  capped off with a 99-yard touchdown return in the bowl game. He finished the season with seven returns of 39 or more yards, including an 81-yarder at Indiana and a 74-yarder at Wisconsin . Against the Badgers, he racked up 206 yards on five kickoff returns to break 42-year-old school records for total yardage and average yards per return in a single game. (41.2)  For his efforts against Wisconsin he was named the Big Ten Freshman of the Week, becoming the first freshman to do it since Rob Henry in October 2010.  His 837 kickoff return yards are the second best in school history for a single season. He is now ranked 13th on Purdue's career kickoff return yards.
In May, 2012, Mostert was named to the preseason Jet Award watch list, an award given to the top return specialist in the NCAA.  During spring practice, Mostert was named a captain for the 2012 season . 
|NCAA Career Totals||136||759||5.6||18.5||6||19||122||0||88||2,289||26.0||2||100|
After going undrafted during the 2015 NFL Draft , Mostert signed as a undrafted free agent with the Philadelphia Eagles .  Mostert had an excellent preseason, between his 157 rushing yards and 191 receiving yards, totaling for 351 yards, he led the NFL in yards from the line of scrimmage in the preseason. He also added in 5 kick returns for 162 yards. Despite this effort, he was ultimately cut on September 4, 2015, in preparation for the Eagles having to cut their roster down to 53. 
On September 6, 2015, Mostert was signed to the Philadelphia Eagles' practice squad.
On September 14, 2015, The Miami Dolphins signed Mostert off the Eagles practice squad. Mostert returned two kicks for 57 yards for the Dolphins during Week 2. On October 13, 2015, he was released by the Dolphins in hopes of moving him down to practice squad. Mostert did not clear waivers and was picked up by the Baltimore Ravens the following day.
On October 14, 2015, Mostert was signed by the Baltimore Ravens after losing third-string running back, Lorenzo Taliaferro to a season-ending foot injury.  Mostert returned 5 kicks for 164 yards in 7 games with the Ravens. On December 15, 2015, Mostert was cut by the Ravens in hopes of putting him on practice squad; however, Mostert did not clear waivers and was placed on the Browns 53-man roster. 
New York Jets
On September 12, 2016, Mostert was released from the Jets' practice squad. 
On September 13, 2016, Mostert was signed to the Bears' practice squad. On September 21, 2016, he was elevated to the active roster.  He was released on October 3, 2016 and re-signed to the practice squad the next day. He was released by the Bears on November 24, 2016.
San Francisco 49ers