Posthumus attended River East Collegiate, a high school located in Winnipeg, Manitoba in Canada. He was considered one of the top high school basketball players in the country as a senior, averaging 39 points, 25 rebounds, and 7 blocks. His career-high with River East was a 69-point performance. Through his high school years, Posthumus led his team to two provincial quarterfinals and was named a provincial All-Star in his final two seasons with the Kodiaks.
Collegiate career
Posthumus played with the Thunderbirds of the University of British Columbia for his freshman college basketball season. In a total of 16 games played, he averaged 1.7 points and 2.8 rebounds. Nevertheless, his team finished the season with a 25–3 record and put together a 13-game winning streak as well. Posthumus did not play basketball in his true sophomore season as a redshirt and would end up transferring to Howard College in Big Spring, Texas in the United States the following year. At Howard, the center appeared in 24 contest and averaged 4.0 points and 3.2 rebounds. He shot.570 on field goals and.690 from the free throw line. The Hawks finished the season with a 23–8 record. Posthumus then transferred to Morehead State, in NCAA Division I as a redshirt junior. He played in all 33 of the Eagles' games and averaged 7.5 points and 6.8 rebounds. Posthumus also recorded two double-doubles and had career-highs of 19 points and 19 rebounds against Oakland and SIU Edwardsville respectively. Posthumus returned to Morehead State for his final college season and averaged 9.6 points off 51% shooting along with 10.9 rebounds per game which was the second-highest in the nation. He also posted 10 double-doubles and following the season Posthumus earned College Sports Madness All-Ohio Valley Conference honors and gained fame back in Canada.
Professional career
Over the summer after his graduation, Posthumus signed with Chicago Bulls and participated in the NBA Summer League. On August 1, 2014, Posthumus signed his first professional contract with Levanga Hokkaido of the Japan Basketball League. Posthumus made his pro debut on October 11, 2014, against the Mitsubishi Diamond Dolphins, scoring 16 points and posting 13 rebounds. On November 23 and 24, in games vs the Tsukuba Robots, he recorded back-to-back double-doubles, grabbing a season-high 15 rebounds in the first contest. He finished his stint with the Japanese team averaging 10.1 points and 8.1 rebounds in 23 minutes per game. On January 9, 2015, Posthumus inked with the Mississauga Power of the National Basketball League of Canada. He debuted with the Power by scoring 34 points and posting 22 rebounds. In all three of his games, Posthumus put up double-doubles, but he failed to lead his team to any wins. On January 22, 2015, Posthumus signed with Argentino de Junín of the Liga Nacional de Básquet in Argentina for the remainder of the season where he averaged 9.7 points and 4.9 boards. On September 3, 2015, he signed with the London Lightning and made a return to the NBL Canada. He would play under former Power head coach Kyle Julius, who left the Power after the organization folded. On February 27, he was acquired by the Island Storm in exchange for the rights to Nick Evans. Posthumus signed with the Saskatchewan Rattlers of the CEBL in 2019. He averaged 5.2 points and 4.6 rebounds per game. Posthumus re-joined the team the following year
