A first-round pick (No. 20) by the Devils at the 2004 NHL Draft, Zajac helped the University of North Dakota reach the NCAA Frozen Four in back-to-back seasons (2005, 2006). He was named to the NCAA Championship All-Tournament Team, the NCAA East All-Tournament Team and WCHA All-Rookie Team as a freshman in 2005.

A first-round pick (No. 20) by the Devils at the 2004 NHL Draft, Zajac helped the University of North Dakota reach the NCAA Frozen Four in back-to-back seasons (2005, 2006). He was named to the NCAA Championship All-Tournament Team, the NCAA East All-Tournament Team and WCHA All-Rookie Team as a freshman in 2005.

Zajac signed with the Devils on April 13, 2006 and made his NHL debut against the Carolina Hurricanes on Oct. 6. He scored his first NHL goal the next night against the Dallas Stars.

He set NHL career highs in goals (25) and points (67) in 2009-10 and played in a Devils record 401 consecutive games between Oct. 26, 2006 and April 10, 2011, passing the previous mark set by defenseman Ken Daneyko (388).

Zajac missed most of the 2011-12 season with a leg injury but returned for the Stanley Cup Playoffs and scored 14 points (seven goals, seven assists) in 24 games, helping New Jersey reach the Final, where it lost to the Los Angeles Kings.

The Devils signed Zajac to an eight-year contract extension on Jan. 16, 2013.

He reached double digits in goals in six straight seasons and had nine when the 2019-20 season was paused March 12 due to concerns surrounding the coronavirus.