Lappalainen was born in Helsinki, Finland, in 1907. He studied at the Helsinki University of Industrial Arts and worked for several Finnish furniture manufacturers, including Asko and Korhonen.
In the 1940s, Lappalainen began experimenting with organic forms and natural materials, such as bent plywood, to create modernist furniture. His "Lilja" chair, with its lily-shaped backrest, became emblematic of Finnish design in the 1950s.
Lappalainen also worked on interior design projects, notably for architect Alvar Aalto's famous Finlandia house. In the 1960s, he created plastic furniture for the Finnish brand Iskun Kaluste Oy.
Lappalainen won several awards for his designs, including the Pro Finlandia prize in 1957. He died in 1997 at the age of 90. Today, his modernist wooden furniture is highly sought-after by collectors and design enthusiasts.