After playing four seasons with the Tigers ice hockey team, graduating with a Bachelor's degree in economics, he was signed as an undrafted free agent by the New York Islanders on a two-year contract.

On January 26, 2010, whilst playing in a home game at Nassau Veterans Memorial Coliseum against the Washington Capitals, Hillen took a direct hit to the face from an Alexander Ovechkin slapshot.

[1] On October 3, 2013, a hit from Calgary Flames forward Lance Bouma rammed Hillen into the boards, where he fell awkwardly and had to be helped off the ice after being attended by trainers for several minutes.

In the midst of his third season with the Capitals in 2014–15, Hillen produced five points in 35 games as a depth defender before he was traded, along with a fourth-round pick in 2015, to the Carolina Hurricanes in exchange for Tim Gleason on February 28, 2015.

[5] As a free agent Hillen was unable to attain an NHL contract, and having sustained several injuries opted to retire from professional hockey midway into the following season on December 30, 2015.