Following his parents' divorce, Roberto and his mother relocated to New York, where he attended high school and college.

[1] In 2004, Mandje qualified for the 1500m and 3000m steeplechase, representing Equatorial Guinea at the Athens Olympic Games.

During the first-round heat of the 1500m, which included world record-holder Hicham El Guerrouj of Morocco (the eventual Olympic champion), Roberto was tripped early in the race.

From 2004 to 2016, Mandje competed professionally in track, road and trail races in the United States, Africa and Europe.

Personal bests: 1500m — 4:00.33 (Ninove, BEL), 11 AUG 2007 3000m SC — 9:04.54 (Los Angeles, CA, USA), 7 JUN 2008