Born to Icelandic parents who were students in Mobile, Alabama, United States, Aron moved to Iceland with his family at the age of three.[2] Growing up, he spent most his life in Iceland while occasionally visiting the United States as well as spending one year and graduating in 2008 from The Pendleton School at the IMG Academy in Bradenton, Florida in the United States.[3]
Aron made his debut for Fjölnir in the 2008 Úrvalsdeild season. He became a regular in the side the following year, making a total of 37 league appearances for the club and scoring 13 goals in the process. 12 of those he scored in his last season with Fjölnir, when he was voted both the best and the most promising player of the first division. On top of that he was also the top scorer of the league.
AGF Aarhus
Aron attracted attention from foreign clubs during his second full season for Fjölnir, moving to AGF Aarhus in late August 2010. Following a fine performance in AGF's 3–2 win at Viborg FF on April 16, 2011, he was chosen as one of the team's starting strikers. Aron played the remaining games for AGF in the season, helping his team to the promotion to the Superliga. His first goal for AGF came in an away match in the Danish 1st Division against Hvidovre IF on April 28, 2011.
He made his debut in the Danish Superliga on July 18, 2011, in a 2–1 win against Lyngby BK. After 15 goalless matches, he finally scored his first goal in the Superliga in the away match against SønderjyskE on November 7, 2011, securing a 1–1 result.
Aron set a new record August 27, 2012 when he scored the fastest hattrick ever in the Danish Superliga, completing the hattrick in 3 minutes and 50 seconds against AC Horsens; he also scored their fourth goal of the match, giving AGF a 1–4 victory. It took him only 16 minutes to score the four goals, which is another record in the Superliga.[5]
AZ Alkmaar
Aron joined AZ on January 29, 2013.[6] He scored his first goal for the club on April 14, 2013.[7]
On August 11, 2013, Aron scored from the penalty spot in a 3–2 win over reigning Eredivisie champions Ajax.[8] Against Sparta Rotterdam he scored a hat trick in the Dutch Cup on September 25.[9] Three days later, he scored the winning goal in a 2–1 win over PSV Eindhoven.[10] On October 20. Aron netted a brace as AZ came from a goal back to record a 3–1 win.[11]
Werder Bremen
2015–16 season and long injury layoff
On August 5, 2015, Aron underwent a medical and signed for Werder Bremen on a four-year deal, for a fee reported to be in the region of €5 million.[12] Just ten days later, on August 15, he made his debut replacing Levin Öztunalı after 57 minutes in Werder's first match of the 2015–2016 Bundesliga season, a 0–3 home defeat to Schalke 04.[13] He made his full debut a week later in a 1–1 draw away to Hertha Berlin on the second matchday of the season,[14] before scoring his first goal for Werder in his team's 2–1 defeat of Borussia Mönchengladbach on August 30, 2015, converting a penalty for 1–0.[15] He picked up his second goal at Darmstadt 98 on September 22, the opening goal in a 2–1 defeat.
In early October, Aron was diagnosed with a nerve irritation in his right hip which Werder Bremen announced they would treat "in a conservative manner" and which would keep him out of action for a minimum of three weeks.[16] On October 28, he underwent hip surgery that would leave him unable to play for "the coming weeks".[17] However, after multiple attempts at a comeback,[18][19] his season was ruled to be over by club CEO Thomas Eichin on March 24.[20] Coming into the club with high expectations, Aron was not really able to prove his worth during his first season. He made six appearances before his season-ending injury, scoring two goals.
2016–17 season
After being expected to miss the beginning of the 2016–17 season as well,[21][22] Aron started in the opening match, a 6–0 defeat away to Bayern Munich, returning after an eleven-month layoff. He scored his first goal of the campaign in his second appearance, converting a penalty in a 1–2 defeat to FC Augsburg. The following week, in a substitute appearance against Mönchengladbach, Aron assisted Serge Gnabry's goal. However, he was sent off for cursing, in English, at the referee, for which he was banned two matches and fined €8,000.[23] In the days following the incident, Aron claimed he was misheard, but when he tried to explain to the referee his version, he said that the referee "stayed by his opinion".[24] In addition, that was manager Viktor Skrypnyk's last game in charge, before he was sacked and replaced with reserve manager Alexander Nouri.[25] Following Aron's suspension, he began losing playing time to young center forward Ousman Manneh, and acknowledged at the end of his suspension that a return to the starting lineup would "certainly not be simple".[26]
With Aron yet to make a start under Nouri at the end of October, assistant coach Markus Feldhoff voiced his concerns about his match fitness, which Aron argued was "very near 100%", regarding his recent return from a lengthy injury.[27] After making two substitute appearances in February in defeats against Augsburg and Borussia Mönchengladbach, he did not feature in Werder's following four matches during which the team amassed ten points.[28]
2017–18 season
Until Nouri's dismissal in October 2017, Aron hardly featured and often did not make the squad[29][30][31] playing a total of 80 minutes in seven substitute appearances.[32]
During the 2017–18 winter training camp, new manager Florian Kohfeldt praised Aron's competitive spirit.[33] On February 3, 2018, Aron was involved in two goals in Werder Bremen 2–1 win away to Schalke 04 after coming as a substitute,[34][35] This performance earned him a place in the starting lineup in the DFB-Pokal quarter-final match against Bayer Leverkusen three days later. He scored for 2–0 but Werder Bremen lost 4–2 after extra-time.[36][35][37] From mid-March, he was repeatedly kept out of action with hamstring problems.[38][39][40] In early May, Werder Bremen announced Aron would miss the rest of the season due to injury.[41]
2018–19 season
Aron was sidelined with an ankle injury in July 2018[42] und underwent surgery in November.[43] He returned to team training in late February 2019.[42] On the last matchday of the season, he made his farewell from the club during the pre-match ceremony and was given a short substitute appearance by manager Kohfeldt. In his four seasons at the club, he made a total of 30 appearances across all competitions scoring five goals while being repeatedly kept out of action by injuries.[44]
In 2020, Aron was Hammarby's top scorer in Allsvenskan, scoring twelve goals in 22 appearances, although the side disappointedly finished 8th in the table. He decided to terminate his contract with the club at the end of the year.[47]
Lech Poznań
Free agent Aron joined Ekstraklasa club Lech Poznań in February 2021, having agreed a contract until the end of 2021 with the option of an 18-month extension until summer 2023. At the time, the club placed 10th out of 16 teams in the league.[48] He started and scored a game-winning goal in his debut game against Śląsk Wrocław on February 21.[49]
Having picked up an arm injury in a reserve team's league game that would keep him sidelined for the rest of the year,[50] he left the club by mutual consent on the last day of the summer transfer window.[51]
Valur
On 4 November 2021, Jóhannsson returned to Iceland to sign with Úrvalsdeild side Valur.[52]
Aron's decision to choose the United States over Iceland was widely criticized in his home country. In a written statement The Football Association of Iceland (KSÍ) urged the player to reconsider his decision, claiming that the player – who grew up in Iceland – had "no link to soccer in the USA at all" and that there was no logic behind him "relinquishing his Icelandic football identity".[56]
Aron made his first appearance for the United States in their 4–3 friendly victory over Bosnia and Herzegovina in Sarajevo the next day, coming on for Eddie Johnson in the 63rd minute. In the final World Cup qualifying game against Panama on October 15, 2013, he scored his first goal in the final minute, securing a 3–2 win.[57] He was included on Klinsmann's 23-man squad for the 2014 FIFA World Cup, coming on for an injured Jozy Altidore in the 23rd minute of the opening game against Ghana for his tournament debut.[citation needed]