In January 2026, was named the Breakthrough Sportsperson of the Year at the Idrettsgallaen, Norway's Sports Gala. The annual award is presented to the athlete who delivered the most impressive breakthrough on the international stage over the previous year.

Father, Magnus, was named CEO of Reitan Kapital, the financial investment arm of REITAN, in 2016. Magnus attended the NHH Norwegian School of Economics, BI Norwegian Business School, and Florida International University.

Grandfather, Odd Reitan, is the CEO and co-owner (with sons Ole Robert and Magnus -- the latter being Kristoffer's father) of REITAN, a Norwegian conglomerate with businesses in three sectors: retail (Reitan Retail, with Ole Robert serving as CEO), real estate (Reitan Eiendom, with Odd serving as the chair), and financial investment (Reitan Kapital, with Kristoffer's father, Magnus, as CEO). Odd is also the owner and founder of REMA 1000, a discount grocery store he established in Trondheim, Norway in 1979 and grew to nearly 1,000 locations by the early 2020s -- which in turn led to the establishment of REITAN's real estate business (Reitan Eiendom) in the 1990s.

Odd is the son of Ole and Margit Reitan (Kristoffer's great grandparents), who opened their first grocery shop in Trondheim, Norway in 1948. Odd earned a degree from the Retail Management Program at BI Norwegian Business School and eventually opened his first shop in 1972. Seven years later, Odd founded REMA 1000, the success of which eventually led to the creation of REITAN.

Qualified for the 2018 U.S. Open while still an amateur, becoming the first Norwegian to compete in the U.S. Open. Two months later, Reitan qualified for match play at the 2018 U.S. Amateur Championship at Pebble Beach Golf Links, and he was eliminated in the Round of 16 by fellow countryman Viktor Hovland, who went on to win the title and earned an exemption for the 2019 U.S. Open (where another Norwegian, Andreas Halvorsen, also qualified for the event).

Played half a season (fall 2017) at University of Texas. Continued his amateur career outside the college golf ecosystem for all of 2018.

Alongside fellow Norwegians Viktor Hovland and Kris Ventura, led Norway to a runner-up finish at the 2013 European Boys Team Championship.

Helped Norway win the team title at the 2014 Toyota Junior World Cup, where his teammates included Viktor Hovland. Norway notably held off a United States team led by PGA TOUR winner Cameron Young and Korn Ferry Tour graduate Zach Bauchou. Reitan and Hovland also represented Norway the following year at the 2015 Toyota Junior World Cup.

Represented Norway at four European Boys Team Championships (2013, 2014, 2015, 2016), the 2017 European Amateur Team Championship, and the 2018 World Amateur Team Championship (also known as the Eisenhower Trophy).

Earned medalist honors in stroke play at the 2016 Boys Amateur Championship, but lost in the third round of match play.

Earned medalist honors in stroke play at the 2015 European Boys Team Championship and 2017 European Amateur Team Championship.

Represented Europe at the 2016 Junior Ryder Cup.