I first met Mark in Berlin, very soon after the Second World War. In 1950, we were working together - I with the BBC, he for the Manchester Guardian. Mark was untypical of the foreign correspondents of those days; self-effacing, modest, a loner when pursuing a story. Never one of that often hard-drinking crowd, he stuck out because one had the feeling (at least I did) that the stories he was writing were not those others were chasing. He would go off to pursue a subject on his own.
No one knew that at the age of 21 he had been a naval officer, operating out of Dover. I met him briefly in 1944, as a passenger on his night patrol across the Channel. He never spoke of his wartime experiences and made the switch to civilian life without hesitation.
He was labour correspondent at The Guardian before moving to The Observer as political correspondent. In 1963, there was a Tory leadership crisis. Prime Minister Harold Macmillan, ill and unable to attend the party conference, had resigned and, without consulting his cabinet, recommended Alec Douglas-Home to the Palace. Mark, for The Observer, predicted that Douglas-Home would get the nod. His editors changed Mark's copy to read Rab Butler, which turned out to be wrong. Mark resigned in disgust - and after a spell without a job returned to The Guardian as chief leader writer.
Mark later moved to ITN, as deputy to editor Sir Geoffrey Cox, making the switch to television without any difficulty. In the 1970s, he wrote the series The World at War, with narration by Laurence Olivier and Jeremy Isaacs producing.
We lost touch, and our paths rarely crossed again. He died of cancer at 61. His name was forgotten, but not by those who knew him. He was the kind of person I would like to have been; that's why I describe him as my mentor.
Charles Wheeler celebrates 60 years as a BBC correspondent next year