In 2006, rowing a single scull, he won a gold medal at the World University Games.

"[9] After graduating high school, he began studying for an engineering degree at the University of Canterbury.

[17] In 2007, Cohen won a silver medal at the United States World Rowing Challenge in Oklahoma in men's single sculls.

He won a gold medal in the men's double sculls with rowing partner Joseph Sullivan at both the 2010 (at Lake Karapiro, by six hundredths of a second over Germans Hans Gruhne and Stephan Krüger, in 6:10.76) and 2011 World Rowing Championships (in Slovenia, in 6:22.63).

[23][8][21][22] In 2011, he won the 2011 New Zealand National Rowing Championships single scull title at Lake Ruataniwha in Twizel, with a time of 7 minutes, 3.97 seconds, to win the Sir Bernard Freyberg Cup.

[28] At the 2012 Summer Olympics at Eton Dorney, Windsor, Cohen and rowing partner Sullivan won the gold medal in the men's double sculls, after a training regimen of 200 km each week on the water.

[23] They finished ahead of Italy's Alessio Sartori and Romano Battisti by 1.13 seconds, and Slovenian 2000 Olympic champions and 2004 silver medalists Luka Špik and Iztok Čop came in third.

[36] In the 2013 New Year Honours, Cohen was appointed a Member of the New Zealand Order of Merit for services to rowing.

[2] During training in April 2013, Cohen suffered an irregular heartbeat, later diagnosed as supraventricular tachycardia (SVT).

[40][41][42] That August, he withdrew from the World Rowing Championships in Chungju, Korea, due to a recurrence of the problem during preliminary heats.

[46] In 2019 he competed in the fours with his brother Hayden, Joe Findlay and Italian Daniele Danesin.