Flagstad was raised in a musical family surrounded by future jazz talents like Einar Schanke and Bonsak Schieldrop at Ris, Oslo, and became a professional musician as a young man (1947).
He led his own Be Bop Band (1949–50), and joined the orchestras of Tage Wilford and Hans Backe (1945).
There after he toured with Per Asplin (1948), Karl Otto Hoff, and contributed in Egil Monn-Iversen's orchestra (1951), before joining the lineup of the Swede Rolf Ericson (1952–54).
His film music for the Line was awarded with Buddyprisen in 1960, and he led his own Mikkel Flagstad Quintet including Atle Hammer (trumpet), Kjell Karlsen (piano), Erik Amundsen (bass) and Ole Jacob Hansen (drums), while he took dentist education.
The disease shortened his career in the early 1970s, then he suffered a stroke in 1990 and died of pneumonia in 2005.