Leif Carlsen was a Danish football forward that represented the Danish Copenhagen side Frem in the Danish top league from 1964 to 1966. During the 1967 and 1968 seasons, he represented Hvidovre IF where he scored 6 goals. One of them was in the Danish Cup final, Carlsen scored the second goal in the 2-0 victory against Esbjerg fB Thereafter he moved across the Øresund to Swedish team Landskrona BoIS. After the 1970 season he became a celebrated hero for his new club. Landskrona BoIS had at the time been outside the finest Swedish football assembly, Allsvenskan for two entire decades. In 1968 their arch rivals Helsingborgs IF had been relegated down to Division 2 for the first time ever. And "HIF" failed to return to Allsvenskan in the 1969 season, due to a lost qualification. But by 1970 it was time for Landskrona BoIS instead. In August 1970 Leif Carlsen represented BoIS in the perhaps most important match ever played at Landskrona IP, primarily as the game most likely would decide which of the arch rival clubs that would win the regional Division 2 South league, Landskrona BoIS or Helsingborgs IF. Secondly for what later would become historic reasons.
In front of 15.015 attenders Leif Carlsen scored the only goal. And now a qualification league for promotion to Allsvenskan awaited for Carlsen and his BoIS. This qualification league comprised four teams, the winners of the four regional Division 2 leagues. Apart from Landskrona BoIS, winner of the southern division 2 league, also Sandvikens IF (eastern), IFK Luleå (northern) and Skövde AIK (western) participated. Only two of the teams would be promoted. The rules stipulated one game should be played at home, one game away, and the last game would be played at neutral ground. BoIS started at home, and defeated Sandviken in front of 15.685 attenders, 2-0. The goals were scored by Sonny Johansson and Tommy "Gyxa" Gustafsson. Then followed a goalless draw in the very northern town Luleå and the last and decisive qualification match was played at neutral ground, Nya Ullevi in Gothenburg. Landskrona BoIS that had missed qualifications in 1958, 1959, 1962 and 1968 now got nervous, as Skövde scored twice. However Sonny Johansson broke "the spell" and scored 1-2, followed by an equalizer from Leif Carlsen.
So, Leif Carlsen contributed in the highest degree to both Hvidove's Danish cup gold in 1967/68 as well to the promotion of Swedish Landskrona BoIS to Allsvenskan in 1970 However non-Swedish citizens were at the time not allowed to play in the top Swedish league, hence Leif Carlsen couldn't represent Landskrona BoIS in 1971, much due to his own efforts for the club! While Landskrona BoIS made a rather successful return to Allsvenskan in 1971, their arch-rivals Helsingborg IF was yet again relegated down to Division 3 in 1971, and wouldn't play in Allsvenskan again for another 21 seasons. Leif Carlsens single decisive goal at Landskrona IP in September 1970, hence grew in importance by time and in retrospect.