Matthias Steiner was born in Vienna, Austria. He hails from Obersulz in Lower Austria, where he attended Volksschule (primary school), then Hauptschule (secondary school). Steiner completed an apprenticeship in plumbing.
He started weightlifting in 1995. His father, Friedrich Steiner, a 20-time IWF-Masters World Weightlifting Champion, and also ranks first on the IWF 2008 Hall of Fame Survey of Leading Master Lifters.
Since an untreated infection Steiner suffers from diabetes, diagnosed at his 18th birthday.Before the diagnosis, the first symptom was an intense thirst, then he lost appetite, and 5 kilograms (11 lb) body weight within three months.[3] He went to the doctor when his sight deteriorated. Despite the diagnosis he followed his dream to become a weightlifter.

In 2004, a German woman from Zwickau in Saxony had watched Steiner participating in weightlifting contests on TV. She kept asking the Eurosport commentators for his email address, until they gave it to her. She contacted Steiner, and he agreed to meet her in Lower Austria. They married shortly thereafter and hence he moved to Germany, where he then applied for German citizenship.

On July 16, 2007 his wife Susann died in a car accident. Despite the tragic loss, and after losing 7 kilograms (15 lb) or 8 kilograms (18 lb) body weight, he was able to continue his training.

Steiner received German citizenship in early 2008, and started to compete internationally again, for Germany. On January 23, he won the Beijing Pre-Olympic Tournament with a total of 423 kg.

The biggest success of Steiner's career came in August 2008, when he won the gold medal at the 2008 Summer Olympics. After World record holder Hossein Rezazadeh had retired in July 2008, the event turned into a close competition between Steiner, Evgeny Chigishev, and the tournament favorite, European and World champion Viktors Ščerbatihs.

During the Olympic competition, Steiner failed in his third snatch attempt, and with 203 kg ranked fourth after the snatch. Ščerbatihs was in position to take the lead in the clean and jerk, but failed in his second attempt. Steiner also failed in his first clean and jerk attempt, and Chigishev took the lead with 250 kg in the clean and jerk. Ščerbatihs failed in his third attempt, which would have forced Steiner to increase his last attempt to 259 kg in order to win gold. With a successful clean and jerk of 258 kg, and a total of 461 kg, Steiner became Olympic gold medalist.

Steiner stood on the medalists' podium with a photo of his late wife, who would otherwise have accompanied him to the Games.