Najmul Hossain is a classic left-handed batsman who started playing cricket at a very young age. A double-ton to his name at the school level grabbed enough eyeballs and immediately led to his consideration for the 2014 Youth World Cup when he was just 15 years old. Soon after that, Najmul struck his maiden domestic century while representing the Rajshahi Division.

Things turned pretty for the southpaw when he was asked to play another U-19 World Cup in 2016. In the tournament, he received two back-to-back Man of the Match awards against South Africa and Scotland respectively. Hossain emerged as one of the standout performers of the tournament, amassing 259 runs, and also helped his team reach the third position. Hossain's form continued in the Bangladesh T20 League as well and he went ahead to score a brilliant fifty on his debut representing the Comilla Victorians team.

Brilliant performances at the U-19 level followed by some fine domestic and Bangladesh T20 League stints were enough to impress the selectors and with immediate effect, he was named in Bangladesh's preliminary camp held in Sydney before the tour of New Zealand. Hossain was handed his Test cap in the second Test match against the Kiwis as a replacement of the injured Mominul Haque. Najmul had a mediocre debut as he managed to score a mere 18 and 12 with the bat and eventually he was dropped from the squad.

In the following years, Najmul was handed few international opportunities but failed to make proper use of it and thus was sent back to the domestic circuit to enhance his skills and work on his consistency. In September 2019, after Bangladesh sustained a humiliating loss at the hands of Afghanistan during a tri-nation T20I series, Najmul was called-up for the remainder of the series.