He attended Junípero Serra High School in San Mateo, California, where he excelled in basketball and baseball.
He decided to forgo his senior year at American University and pursue his dream of playing professional baseball.
In his career at American University, Lumpkins averaged 13.6 points, 8.7 rebounds, 1.3 blocks, and 1.1 assists in 31.1 minutes per game.
In rookie ball in 2011, Lumpkins posted a 7.02 earned run average in 8 starts in 10 games with a farm team in the Royals organization.
Lumpkins played alongside NBA professionals Jeremy Lin and Diamon Simpson, who both spent time in the Houston Rockets organization.
Lumpkins played in this event after going undrafted, prior to signing for Aris Leeuwarden in order to hone his basketball skills.
In a close 97–98 loss to Rotterdam, Lumpkins scored 29 points on 11-13 shooting, while going 7-12 from the free throw line, to go with eight rebounds, four assists, and one steal.