Karjalainen began karting at the age of thirteen in 1999, entering the Finnish Championship Junior A. He continued through successive Finnish national karting categories and progressed to international competition by 2002, racing with Ward Racing in a Tony Kart chassis. In 2003 he raced for PDB Racing Team, taking a podium with third place at the Coupe de la Ville de Braga Formula A in Portugal and competing in both the Nordic Championship and the FIA European Championship Formula A.
Karjalainen moved into single-seaters in 2004 with Matson Motorsport in the German Formula BMW ADAC series, contesting all 20 events and finishing 24th with no points. For 2005 he joined Team Rosberg and accumulated 5 points, including a career-best eighth at Hockenheim, to finish 21st.
Seeking greater opportunities, Karjalainen relocated to Asia for 2006 with the Asia Racing Team in the Asian Formula Renault Challenge. He won at Zhuhai International Circuit and took three further podiums, two pole positions, and one fastest lap to finish fourth in the championship with 173 points from 12 rounds. He simultaneously sampled Formula 3 Asia competition that year with Team Goddard, finishing 13th overall.
In 2007 Karjalainen elevated his performance significantly in the Formula 3 Asia Pacific Championship with Team Goddard. He won five races, took ten podiums, and set three pole positions and eight fastest laps across 13 events to finish runner-up with 189 points, the strongest single-seater result of his career.
That same year Karjalainen received an opportunity to race in the GP2 Series with BCN Competición at Istanbul Park, filling in for injured Finnish compatriot Markus Niemelä. He qualified 25th and retired from the feature race after 15 laps; he retired from the sprint race on the opening lap after spinning at the first corner. He scored no points and was replaced when Niemelä recovered.
For 2008 Karjalainen moved to North America to race in the Atlantic Championship with Jensen MotorSport, driving a Swift 016.a Mazda. He contested ten of eleven rounds and accumulated 70 points for 17th in the standings. His best individual result was tenth at the Grand Prix de Trois-Rivières.
In 2009 Karjalainen entered the reformed FIA Formula Two Championship with MSV Racing, piloting the No. 23 Williams JPH1 with a Mountune Audi engine across 16 races. He scored 7 points for 15th in the standings, with his best finish fourth in the opening race at Brno. Concurrently in 2009 he competed in three rounds of the Formula Renault 2.0 NEC series with Red Step Formula, scoring 31 points and finishing 23rd, with a fourth place at Alastaro his best result.
Karjalainen returned to Finland in 2010 to race with Ferrari Team Nurminen in the Finnish GT Championship, sharing a Ferrari F430 Challenge GT3 Cup car with veteran Jari Nurminen. The pair took eight podium finishes across eight rounds and finished fourth in the championship with 222 points. The team recorded no victories, poles, or fastest laps. This season marked the practical end of Karjalainen's active driving career.
Karjalainen transitioned into motorsport event production and management. He worked at AKK Sports Oy, the commercial subsidiary of the Finnish Automobile Sport Federation, in roles including trackside manager for the Karting World Championship, Neste Rally Finland, and the Finnish Digital Motorsport Championship finals. From 1 October 2019 he served as discipline manager at AKK-Motorsport ry for karting, circuit racing, and digital motorsport. His duties included overseeing events such as the FINEST Cup karting competition and developing structural frameworks for Finnish national motorsport series. He also served as head coach for Pink Panthers, a team in the Finnish basketball leagues in Helsinki.