Willem George Frederik Jansen was born in Harlingen. His talent as a technical draftsman became apparent early on, but it would be some time before he started working as a free artist. In Arnhem, Jansen started training as a blacksmith and fitter at the vocational school. After moving to Haarlem, he befriended a decorative painter. Through him, Jansen discovered his interest in painting and started training as a decorative painter at the Kunstnijverheidsschool in Haarlem. After his education he started with ceiling paintings in Amsterdam, and in 1896 he was appointed as chief designer at the pottery factory 'Rozenburg.
After his marriage to Hildegonda Zijlstra in 1898, he lived between 1899 and 1908 in Loosduinen near The Hague and in Amsterdam. In The Hague, Jansen was close to the places where painters such as Jozef Israels, Anton Mauve and Jacob Maris of the Hague School had worked a few decades earlier. It was Jacob Maris in particular that he admired, which can be seen in Jansen's smooth, recognizable, impressionistic brushwork and his love for the purely Dutch landscape. But in addition to the landscapes with windmills and cityscapes that Maris often made, Jansen also painted sea and beach scenes, intimate farmyards, river views under high skies, meadow landscapes and quiet heath landscapes with sheep. He was widely praised for his highly developed sense of colour nuances, especially in colored whites, greens and greys.
Jansen was a member of Arti et Amicitiae in Amsterdam and the Haagse Kunstkring. In The Hague he befriended Théophile de Bock and became increasingly known for his paintings through his participation in exhibitions. Yet he only managed to make a living from his painting after 1908. In that year, after a visit to Willem Knip in Laren, Jansen decided to move to Het Gooi, where he settled in the former brewery in Blaricum. Until 1920 he would mainly work in the Gooi and he made trips to Brabant and Drenthe. After 1920 he could often be found in Egmond aan Zee. There the family had a house and Jansen could make studies of the fishing boats and shell fishermen on the beach to his heart's content in his own studio.
Jansen was a skilled artist with a balanced temperament, who had many friends among the Laren artists. He was an independent personality who was not easily influenced. His legacy is large, he made thousands of paintings, which are counted among the aftergrowth of the Hague School. He sold well and also had relationships with English, American and Canadian art buyers. Jansen died in Blaricum where he had always lived.