Leon Scieur was the son of a farmer in Florennes, Belgium. He began work as a glassmaker before being introduced to cycling by his neighbor, Firmin Lambot, who taught him to ride a bike at the age of 22.
Scieur turned professional in 1913 and rode his first Tour de France that year, without finishing. He finished 14th in 1914.
After working as a mechanic in World War I Scieur rode the revived Tour de France in 1919 placing fourth. He had punctured four or six times, according to reports, between Le Havre and Cherbourg. The weather was foul and he had no more spares so he had to huddle in a doorway to repair one of the punctures. Mending a race tire involved taking it from the rim, cutting the stitching that held the base together, mending the inner tube, then sewing up the tire before replacing it. Scieur had acquired a needle and thick thread from the woman in whose doorway he was sheltering in, but his fingers grew too cold to use them. He asked the woman to help but the chief official, Lucien Cazalis, told him "It's forbidden to receive help; you'll be penalised if madame threads the needle for you." Scieur completed the repair but lost the Tour to Lambot by about the time it had taken.
He finished forth in the 1920 Tour de France.
He won at his fifth attempt, in 1921, when he was 33. He went into the lead on the second day and rode so hard to defend his position that reporters nicknamed him "The Locomotive." He pedaled fast in a low gear, winning two stages, from Cherbourg to Brest and from Nice to Grenoble.
Another Belgian, Hector Heusghem, attacked when Scieur punctured on the Col d' Allos, which climbs to 2,240 meters. Scieur ws so angry at the breach of etiquette that riders weren't attacked when they had mechanical trouble that he set off after Heusghem, lectured him on politeness and tradition, raced off angrily alone and won the stage to Grenoble. The feud that developed between them brought still more reporters from Belgium - this was the first year that foreign reporters could follow the race by car - and made life hard work for everyone. The organiser, Henri Desgrange, wrote a column in L'Auto criticising riders for being too scared of Scieur to challenge him.
Desgrange wasn't slow to criticise or disciple riders who he thought weren't riding hard enough. The 12th stage was 371 km from Geneva to Strasbourg. Scieur was leading the race with Heusghem and a French rider, Honore Barthelemy. Two Belgians, Firmin Lambot and Louis Mottiat, stayed in the main group rather than chase and spoil Scieur's chances. All five riders were in the same team and were using tactics that today would be considered normal. Desgrange, however, believed riders should compete as individuals and not in teams and he banished Lambot and Mottiat to last place.
The Tour became duller after Heusghem and Scieur settled into a sullen truce, but it wasn't without incident. Scieur broke 11 spokes on the next to last stage, from Metz to Dunkirk and again fell foul of Desgrange's rules. He managed to get a replacement wheel but new rules for that year's Tour said he didn't have the right to use it unless he could show Desgranges's judges that the original was beyond use. No judge saw the incident and so Scieur carried the broken wheel on his back for 300 km to the finish. He said it left a mark on his back for 15 years.
Scieur abandonded the 1922 Tour with a broken fork.
Scieur stopped racing in 1924. He spent the rest of his life in Florennes, where he owned a garage and distribution businesses for coal and gas. He died in 1969.