Kaj Christiansen started his amateur footballing career with
KFUM's Boldklub, but like so many of his prolific KFUM teammates, he eventually moved to a larger
Copenhagen club to make his national breakthrough. Playing in the
centre forward position, Christiansen was seen as the natural successor of Frem's club icon and the most popular Danish footballer of his time, centre forward
Pauli Jørgensen. Rather than Jørgensen's physical style, Kaj Christiansen was a quick player of great anticipation. Just months after his debut for Frem, Christiansen made his debut for the
Denmark national football team in June 1943, the last Danish international game during the
Second World War. He scored two goals on headers in the debut game, a 3–2 win against the
Sweden men's national football team, with Danish
King Christian X in attendance. He was subsequently named 1943 Danish Sports Talent of the Year. Kaj Christiansen played seven seasons as a professional in
French football. He signed with French team
Stade Français in 1948. He played a single season in the club, finishing 10th in the
1948–49 French Division 1 season. He then moved to Division 2 team
Le Havre AC, which he helped finish 2nd in the
1949–50 French Division 2, winning promotion for Division 1. He stayed a further two seasons with Le Havre, helping the club to impressive 3rd and 7th-place finishes in the Division 1. In 1952, Christiansen moved to newly relegated Division 2 team
Olympique Lyonnais. In his second year at the club, Lyonnais won the
1953–54 French Division 2 and secured promotion for Division 1. Kaj Christiansen moved on to Division 2 team
Grenoble Foot 38, for whom he played his last season as an active player. Following his active career, Christiansen turned to the role of football coach. He took a coaching license in France, and was named 1964 Manager of the Year in France. He returned to Denmark, and coached
Randers Freja for some years, culminating in winning the 1967
Danish Cup tournament. He also coached minor team Vorup and his old club BK Frem. In 1969, he and his family moved back to live in
Hyères, France. Here, he coached several French teams, and got a job as a
physical education teacher. ==Honours==