[2] He graduated from the University of Texas with a bachelor's degree in finance in 2010, and relocated to Los Angeles shortly thereafter to train with coach Dave Salo at Trojan Aquatics.

At the 2008 Summer Olympics, Berens won the gold medal in the men's 4×200-meter freestyle relay with Michael Phelps, Ryan Lochte, and Peter Vanderkaay, setting a new world record of 6:58.56.

Berens swam the third leg of the 4×100-meter freestyle preliminaries at the 2009 World Championships in Rome and earned a gold medal when the U.S. team placed first in the final.

At the 2011 Duel in the Pool, a short course meet featuring the United States and European "All-Stars", Berens won both the 100-meter and 200-meter freestyle events.

[7] At the 2012 Summer Olympics in London, Berens again won a gold medal in the men's 4×200-meter freestyle relay, together with Ryan Lochte, Conor Dwyer and Michael Phelps.

He also won a silver medal as part of the second-place U.S. team for the men's 4×100-meter freestyle relay, in which he swam the third leg of the heat in 48.52.

At the 2013 World Aquatics Championships in Barcelona, Berens combined with Conor Dwyer, Ryan Lochte and Charlie Houchin in the 4×200-meter freestyle relay, with the team finishing in first place for the gold medal.