He holds notable wins over former world heavyweight champions Lamon Brewster, Samuel Peter, and Siarhei Liakhovich.

Helenius won the bronze medal in 2000 at the European "cadets" (U 17) championships in Patras; he lost to Croatian Mario Preskar.

At the 2001 Finnish Amateur Boxing Championships, in the super heavyweight division, Helenius lost to eventual gold medallist Janne Katajisto in the first round.

In 2009 Helenius knocked out former British heavyweight champion Scott Gammer and defeated Taras Bidenko with a third round stoppage.

In January 2010 Helenius made the biggest step up so far of his professional career in squaring off against former WBO heavyweight champion Lamon Brewster.

Helenius defended his title for the first time on 27 November 2010 by beating Attila Levin with a technical knockout in the second round.

Trying to build on the momentum of his big win over Lamon Brewster, Helenius signed to fight another former world heavyweight champion in Samuel Peter.

Peter was coming off a 10th-round knockout loss to unified heavyweight champion Wladimir Klitschko in their marquee rematch bout.

On 27 August 2011 Helenius successfully defended his WBO and WBA Inter-Continental titles against Sergei Liakhovich in Munich, Germany, by technical knockout in round 9.

On 3 December 2011, Helenius won the vacant European Championship title (EBU) by beating Derek Chisora by a twelve-round split decision in Helsinki, Finland.

After a year away from boxing, Helenius beat veteran Sherman "The Caribbean Tank" Williams on points on 10 November 2012 in Helsinki keeping his clean record.

[15] On 20 January 2016, Helenius vacated the European belt after refusing to fight mandatory challenger Derek Chisora.

[20] On 17 December 2016 Helenius beat Argentinian Gonzalo Basile in 48 seconds in a lackluster scrab at Hartwall Arena, Helsinki.

On 24 March 2017 it was announced that Helenius would get a much awaited rematch against Derek Chisora at the Hartwall Arena in Helsinki, Finland.

[23] After Chisora rematch was postponed, Helenius went on to face Russian Evgeny Orlov on 17 June 2017 and did gain an easy victory in 6th round.

[31] After almost a year away from the ring, Helenius made his first appearance in the United States, facing Gerald Washington on 13 July 2019.

The former had inflicted his opponent with a swollen left eye, before suffering repeated low blows, which caused Kownacki to be disqualified in the sixth round.

[38] Helenius then faced former WBC heavyweight champion Deontay Wilder with the fight scheduled to occur on October 15, 2022, at Brooklyn's Barclays Center.

[48][49][50] Helenius was given a two-year ban by UK Anti-Doping (Ukad) on 19 July 2024 after failing a drug test following his fight with Anthony Joshua.

He tested positive for clomifene, although the boxer denied intentional doping and blamed the consumption of contaminated egg and chicken.