This last name was originally a typical Scandinavian Patronymic: http://wiki.geni.com/index.php/Patronymics It was only introduced as a last name or family name around 1850-1900.
Derived from the personal name John, this is the most famous name and surname of the Christian World. It was recorded from the 12th century a.d. onwards in every European country, and in a vast range of spellings, of which there are believed to be over fourteen hundred in total. These range from the base forms such as Jon or John (England & Wales), Evan (Wales), Ian (Scotland), Shane (Ireland), Ivan (Russia) and Jean (France), to the Italian Giovanni, Zanni and Zoane, the Polish Janus, the Czeck Jan, Janak and Jansky, to the diminutives Jenkin, Jeannet, Nannini, Zanicchi, and Gianuzzi, the patronymics Johnson, Joynson, Jensen, Jocie, Ivanshintsev, and Ivashechkin. However spelt, all have derived from the biblical Hebrew "Yochanan", which translates as " He who Jehovah has favoured (with a son)". The name became particularly popular after the 12th century when returning Crusaders from the Holy Land often called their children by biblical names in commemoration of the fathers pilgrimage, these then in turn developed into surnames. The earliest recordings of surnames are to be found in Britain and amongst these is Thomas John in the "Hundred Rolls" of the county of Buckinghamshire for the year 1279, and Arnold Johan in the 1280 "Letter Book" register for the city of London. In Germany Walterus filius Johannis appears in the 1323 charters of the town of Vaihingen, whilst the 1344 charters of the city of Friedberg record Baumeister Johannssen. The first recorded spelling of the surname in any spelling is believed to be that of Pertus Johannis, which was dated 1230 a.d.. This is in the charters known as the "Close Rolls" of the county of Suffolk", during the reign of King Henry 111 of England,1216 to 1272.
Søn af jens = Jensen (dansih)
Son of Jens = Jensen
Jensen is the most popular surname i Denmark ((http://www.dst.dk/da/Statistik/emner/navne/popall.aspx)