Mark joined the
Royal Flying Corps as a temporary
second lieutenant (on probation), and was confirmed in his rank on 20 October 1917. In January 1918 he was posted to
No. 24 Squadron on the
Western Front to fly the
SE.5a single-seat fighter. His first aerial victory came at 0830 hours on 18 February 1918, when Mark,
Horace Barton and
Andrew Cowper drove a German
DFW two-seater reconnaissance aircraft down out of control. He scored his second win later that same day, driving a
Pfalz D.III fighter down out of control. The next morning, ten minutes combat saw Mark help Cowper,
Reuben Hammersley, and
Peter MacDougall burn another DFW reconnaissance aircraft and destroy a
Rumpler two-seater. On the 26th, Mark shared with
Ian Donald Roy McDonald,
Herbert Richardson, and four other British pilots in the destruction of a new
Fokker Dr.I triplane fighter. Mark was now an ace. and eventually left the RAF, relinquishing his commission on 9 April 1919. ==Second World War==