Hennessy began his professional career with the San Jose Sharks'
American Hockey League (AHL) affiliate, the
Cleveland Barons, during the
2005–06. He led the Barons in goals (24) and assists (39) for 63 points in 80 games. He was one of only two players on the team to play in every regular-season game. Hennessy holds the franchise records for both goals and points in a season by a rookie and was named the Barons Rookie of the Year for the 2005–06 season. Hennessy was traded with
Tom Preissing to the
Ottawa Senators via the
Chicago Blackhawks on July 9, 2006, in a three-way deal; Ottawa traded
Bryan Smolinski and
Martin Havlát to Chicago, who sent
Mark Bell to San Jose while sending
Michal Barinka and a second-round draft pick in
2008 to Ottawa. Hennessy then spent the majority of
2006–07 season with the Senators' AHL affiliate, the
Binghamton Senators. During his second call up to Ottawa, Hennessy scored his first NHL goal on January 7, 2007, against the
Philadelphia Flyers. On May 6, 2010, Hennessy left the Senators organization after four years, signing a one-year deal with Swiss club
HC Lugano. On July 5, 2011, Hennessy signed a one-year, two-way contract with the
Boston Bruins. He was assigned to their AHL affiliate, the
Providence Bruins, for the majority of the
2011–12 season. In 67 games, he contributed offensively with 41 points before he was recalled to appear in three scoreless games with the Bruins. During the
2014–15 season, Hennessy featured in 27 games with
Neftekhimik Nizhnekamsk before opting for a mid-season transfer to Swedish club
Växjö Lakers on December 30, 2014. After adding 13 points in just 20 games to close out the season with the Lakers, and claiming the Swedish championship, Hennessy signed a two-year contract extension on June 4, 2015. ==Career statistics==