At Granger High School in West Valley City, Utah, Tahi was named the USA Today Player of the Year for Utah as a senior. Tahi rushed for a total of 5,700 yards and set a state record as the all-time leader in rushing yards and for rushing touchdowns
Tahi attended Brigham Young University as a freshman in 1999 and led the team that year in rushing with 445 yards before taking a three year leave for a mission in Jacksonville, Florida as he is a member of The Church of Jesus Christ of Latter-day Saints. Tahi returned to Brigham Young in 2003 and had a career-best 872 yards rushing from scrimmage as a senior and scored 8 touchdowns (6 rushing and 2 receiving).
Tahi was an undrafted free agent out of college and signed with the Cincinnati Bengals in 2006, but was released on final roster cuts and became a member of the team's practice squad.