Born in Cova da Piedade, Abrantes began running at the Núcleo Desportivo Juvenil do Laranjeiro in 2002 where he remained until 2003. He joined Sporting Clube de Portugal in 2004, but left in 2010. At the 2007 Summer Universiade he managed a sixth place in the final 100 m, with a time of 10.53 seconds. He participated in the 2007 European Athletics U23 Championships in Debrecen and was a semi-finalist in both the 100 and 200 m, before going on to claim a silver medal in the relay with colleagues Dany Gonçalves, Ricardo Martins and Yazaldes Nascimento. At the Osaka 2007 World Athletics Championships, Abrantes reached the quarter-finals of the 200 m. As well as achieving the entry standard time for the Olympic Games 2008 he also beat his personal best and national under-23 record of 20.48 seconds. He qualified to compete at the 2008 Summer Olympics, but was eliminated in the opening rounds. The following year he improved his 100 m personal best to 10.19 seconds. He achieved a second-place finish at the European Team Championships in the 200 meters, behind only to Dwain Chambers. At the Portuguese Championships he was runner-up to Francis Obikwelu with a wind-assisted run of 10.13 seconds. He was not in shape to compete in the 200 m at the 2009 World Championships in Athletics, but he did run in the national relay team.
2010
He managed to reach the entry standard for the European Championships in Barcelona, the time being slightly above his personal best. He participated in the 200 meters reaching the semi-finals. In the 4x100 meter relay, along with fellow Ricardo Monteiro, Francis Obikwelu and João Ferreira, he beat the national record to 38.88 seconds. It was the first time a Portuguese team completed the relay race in under 39 seconds.
2011
Having left Sporting Clube de Portugal, he began the 2010/2011 season as an individual athlete. Despite competing as individual, he surprised everyone at the National Indoor Championships by winning the 60 m race, beating fellow countryman Francis Obikwelu. Abrantes set a new personal best of 6.65 seconds. At the 2011 European Athletics Indoor Championships in Paris, he qualified for the semi-finals with a time of 6.65, but did not make the final. He started the outdoor season on 15 May at the University National Championships, opening the season with a time 10.56 seconds. A week later, at the Meeting of Leiria, he won with a time of 10.54 seconds. In early June, he participated in the International Flanders Athletics Meeting in Belgium. In the 100 meters he lined up against the then world champion in the 60 meters Dwain Chambers, taking second place with 10.70 seconds. Chambers finished the race in 10.27 seconds – a heavy headwind of -2.3 m/s affected the race. Later in the same meeting, Abrantes won the 200 meters in 20.90 seconds. On 3 July, Arnaldo Abrantes participated in the Meeting of Chaux de Fonds in the 200 meters finishing second with 20.61 seconds. This result achieved the entry standard for the 2011 World Championships in Athletics in Daegu and 2012 Olympic Games in London.
Personal bests
All information taken from IAAF profile, except 300 metres record.
Competition performances
National Championships
* 1st place Outdoor National Championship 2007 100 m - 10.33
* 1st place Outdoor National Championship 2004 200 m - 21.63
* 1st place Indoor National Championship 2009 60 m - 6.72
* 1st place Indoor National Championship 2011 60 m - 6.65