Najar moved with his family from Honduras to the United States at the age of thirteen, settling in the Washington metropolitan area. Najar attended Thomas A. Edison High School in Alexandria, Virginia and joined D.C. United’s youth academy in 2008. During his time with the academy program, Najar received numerous awards and honors for his outstanding play at right back, being named to the USSF Development Academy Starting XI in 2009. He was also the leading scorer of the 2009 US Developmental Academy Finals in Los Angeles.

Najar signed a professional Generation Adidas contract with D.C. United on 22 March 2010. Once part of MLS, the league and his team became responsible for his education. He was the second player to sign with the first team directly from United’s Academy, after goalkeeper Bill Hamid who signed with the first team in September 2009.

Najar made his professional debut on 27 March 2010, in DC’s opening game of the 2010 MLS season against the Kansas City Wizards, and scored his first professional goal on 28 April 2010 in a US Open Cup game against FC Dallas. Najar was voted MLS Rookie of the Year for the 2010 season, ahead of Tim Ream and Danny Mwanga.

Najar was considered by pundits such as Soccer America’s Paul Gardner to be one of DC United top players already, at the young age of 17. He received interest from European clubs, according to his agent, and was expected to move his services there later in his career, possibly as soon as the end of the 2010 season. However, it was announced in December 2010 that Najar had signed a multi-year contract with D.C. United.

On 6 April 2011 Najar formally announced that he would play for the Honduras national football team, choosing his birth nation rather than wait to be eligible to represent the United States, where he had yet to receive citizenship. He made his first appearance for Honduras on 3 September 2011, against Colombia entering in the 67th minute.

Najar was the only player from the D.C. United’s Academy to be called up to compete in the 2012 Olympics.

Najar was called up to compete for the Honduras national under-23 football team in the 2012 Olympics. Najar on his opportunity to compete in the 2012 Olympics “I can only thank God for this” Najar said in a Spanish-language interview Thursday. “I’ll be representing not only my country, but my family as well. The Olympics are a dream for every player and I get the chance to live it.”

On 7 January 2013, Najar moved to Belgian Pro League champions Anderlecht on a one-month loan. On 30 January 2013, Anderlecht finalised the full transfer of Najar. The reported transfer fee was $3 million.

He made his debut against Cercle Brugge on 2 August 2013, coming on as 79th-minute substitute.

On 21 July 2013 Najar scored his first international goal for Honduras in the knockout stages of the 2013 CONCACAF Gold Cup vs Costa Rica. Najar’s game-winner in the 1–0 victory saw Honduras advance to the semi-finals of the tournament.

On 22 October 2014, Najar opened the scoring for Anderlecht in the second half of their Champions League match against Arsenal but two late goals for the English side ensured a 1−2 defeat for the Belgian champions.

On 17 June 2020, Najar returned to Major League Soccer, signing with Los Angeles FC, who earlier in the year had traded $350,000 in General Allocation Money to Nashville SC in exchange for the first selection spot in the league’s Allocation Ranking.