Jón Páll was born in Hafnarfjörður on 28 April 1960, weighing 4 kilograms (8.8 lb) and measuring 52 centimetres (20 in). He was the first child of Dóra Jónsdóttir and Sigmar Jónsson. He was raised by his mother and foster father Sveinn Guðmundsson. The family moved to Stykkishólmur when he was two. He remained there until the age of nine, when the family relocated to Reykjavík. Growing up, he spent his summers on Skáleyjar and was active as a farmhand. He worked from dawn until dusk, carrying pails of water and assisting his foster father on seal hunts. He took up Glima, a traditional Icelandic form of wrestling, at the age of five and later played football and handball, as well as competing in swimming, middle-distance running, and karate.

Jón Páll was introduced to weight lifting in 1976, and began training at Jakaból in 1978. In 1984 he won the Icelandic bodybuilding title in the +90 kg. class. His achievements in powerlifting also include Icelandic records in the bench press (with 192.5 kg (424 lb), 195 kg (430 lb), 210 kg (463 lb) and 222.5 kg (491 lb)) and the squat (with 320.5 kg (707 lb), 330 kg (728 lb), 342.5 kg (755 lb) and 357.5 kg (788 lb)), but his best performances were usually in the deadlift event, in which he set the European record many times (with 350 kg (772 lb), 352.5 kg (777 lb), 360 kg (794 lb), 362.5 kg (799 lb) and 370 kg (816 lb)) and multiple world records in strongman competition deadlift variations, such as the rectangular handled wheel and one handed deadlift.

Jón Páll was invited to the World’s Strongest Man competition for the first time in 1983, in which he came in second only to Geoff Capes. The following year, at age 24, he defeated Capes and secured the title. During the final armwrestling event, in which Jón Páll was up against him, Capes appeared to be winning, pulling Jón Páll’s arm down convincingly, but sustained a muscle tear in his forearm as Jón Páll started to thrust his arm back. Right after winning the bout, Jón Páll shouted “The King has lost his crown!” and won his first World’s Strongest Man title in 1984. Although Jón Páll was closely defeated by Capes at the 1985 World’s Strongest Man, he managed to regain the title in 1986. During the deadlift event at the 1985 World’s Strongest Man competition, someone in the audience called him an Eskimo. Jón Páll shouted back: “I am not an Eskimo. I am a Viking!” and successfully lifted the 495 kg (1,091 lb) cart.

In 1986, Jón Páll first wrestled English author and Guinness World Record Holder Brian Sterling-Vete in a demonstration match for the TV news and print media held at Finnur Karlsson’s gymnasium in Reykjavík, Iceland. Halfway through the match Jón and Sterling had plotted to surprise the audience with a supposed angry outburst leading to the two of them demonstrating their skills as martial artists. This showmanship became synonymous with both Jón Páll and Sterling.

In 1987, Jón Páll clashed with his arguably greatest rival – 3 times World’s Strongest Man winner Bill Kazmaier of Burlington, Wisconsin, USA, who had not been invited to compete at World’s Strongest Man again after winning the competition 3 times in a row from 1980 to 1982. Kazmaier boasted some of the heaviest powerlifting lifts of that time including world records in the bench press with 300 kg (661 lb), deadlift with 402.5 kg (887 lb) and total 1,100 kg (2,425 lb) and had made his reputation in the 1980s as “the strongest man who ever lived” by breaking numerous strongman world records.

After battling with the injuries sustained during 1988, 1989, and 1990, which had affected his athleticism, his strength became progressively more static in his later years. Jon Pall was often challenged by fellow competitor Bill Kazmaier in feats of statically oriented events between contests because he believed the events were biased in Jón Páll’s favour. This included Kaz challenging Jon Pall to a sledgehammer hold during Pure Strength 1987 where Jón Páll beat Kazmaier. Kazmaier had travelled to Nigeria for a strength exhibition with Douglas Edmunds to break the deadlift world record by deadlifting 414 kg (913 lb) with straps, Jon Pall then re-broke the record with a lift of 427.5 kg (942 lb) in his gym in Iceland in front of spectators in 1987 at his heaviest bodyweight in preparation for Pure Strength 1987.

At the 1988 World’s Strongest Man the two rivals clashed again. As expected Kazmaier dominated the static events while Jón Páll, who had shed body-weight to cater for all the dynamic tests of strength of World’s Strongest Man instead of the latter statically orientated events of Pure Strength 1987, was often victorious in the more athletic, speed- and endurance-oriented events. Although Jón Páll was beaten by Kazmaier in the deadlift, log lift and sack race, he managed to win the “weight over the bar event” and the McGlashen Stones in the end to secure the overall victory and to become the World’s Strongest Man for the third time, equaling Kazmaier’s record. After his victory Jón Páll said: “I may be the fastest strongman in the world, but I think Bill [Kazmaier] is the strongest on his feet.”

Jón Páll was also one of a few strongmen who possessed the genetics that allowed him to master all forms of strength. He was able to master the Olympic weightlifting lifts as well as exert tremendous static strength that allowed him to defeat some of the greatest powerlifters of all time. This combination of strength manifested well in World’s Strongest Man, leaving no chinks in his armour. Coupled with strength training he also achieved feats in the Bodybuilding world. This included his famous bodybuilding and strongman wins in 1988, where he had to contrast his heavy strongman training and bulk of up to 140 kg (309 lb) with a cutting up phase to 110 kg (243 lb) for the bodybuilding contests. His weight remained consistently around 130 kg (287 lb) for the World Strongest Man contests, where versatility would be compromised by heavy bodyweights, a point emphasized by Ab Wolders of the Netherlands in 1989, a fellow competitor. Jón Páll weighed his heaviest during Pure Strength 1987 at a weight of 143 kg (315 lb).

After a disappointing third place at the 1989 World’s Strongest Man, Jón Páll was able to win the competition, while injured, for a record breaking fourth time in 1990. O.D. Wilson, who was leading the competition with a comfortable 5½ points before the last event (a 200 m race with a 100 kg weight on the back), weighed 180 kg (397 lb) and lacked the endurance and running speed to complete the course quickly enough and ended up losing by half a point to the much lighter and faster Jón Páll.

The 1990 World’s Strongest Man proved to be Jón Páll’s last appearance on the World’s Strongest Man stage, as he was surpassed as the Icelandic number one by fellow countryman Magnús Ver Magnússon, who won the Iceland’s Strongest Man title in 1988 and 1989 and continued Sigmarsson’s legacy by emulating his 4 World’s Strongest Man title wins during the 90’s.

On 16 January 1993, Jón Páll died of an aortic rupture while deadlifting in his gym, Gym 80 in Reykjavík. This was likely the result of a congenital heart defect that affected other members of his family, which may have been exacerbated by his use of anabolic steroids. He had sought medical treatment in the United States the previous year due to heart problems. In Sölvi Tryggvason’s 2013 biography of Jón Páll he stated he suspected Jón Páll knew he was close to death.

Hjalti Árnason, a lifelong friend of Jón Páll’s, created the Jón Páll Sigmarsson Classic international strongman contest in 2010 in honor of Jón Páll. The event is held annually during the Icelandic fitness & health expo in Reykjavík, the inaugural winner was America’s Brian Shaw in 2010.