The team Fear Youth, who also include Molly Spark and Josh Pearson, finished the race about 5.30pm local time on Wednesday last week, more than 150 hours after starting.
The competition was the first international race for the team, whose members are aged between 18 and 20.
Mitchell said the racing conditions were hot.
"Real hot, and the landscape was so vast and we got to see the scale of the country."
Southland farmers Beacon and Michelle Mitchell travelled to South Africa to watch their son compete.
Mrs Mitchell said it was tough watching Fear Youth battle the heat to finish the race.
"It was epic — we are beyond proud of them."