He was runner-up in the 1911 Amateur Championship, losing to Harold Hilton in the final.

Lassen qualified for the Open Championship five times between 1909 and 1914.

[2] He beat José Raúl Capablanca in a simultaneous exhibition in Bradford.

[4] He was a stuff merchant by occupation, although he got into financial difficulties in the late-1920s.

[2] Note: Lassen only played in The Open Championship.