World War IIHome1976 - 1977
Almost nothing is known about the years 1947 to 1955.
It's unknown when he joined the Secret Service or for whatever organization he is working. Maybe he joined the "Ministry" soon after the war, as he worked in the year 1947 with Peter Borowski, the double agent from series 3 episode Man With Two Shadows.
On the other hand, it's possible that he did first join The Guards when the war was over. He mentions it in The Murder Market in Lovejoy's office, but there's the second proof for it in The Charmers. When he's freeing Kim, he's looking at the tie her hands have been tied with and says, "my old regiment," and it's not the tie from The Intelligence Corps but could be the 4th/7th Royal Dragoons regiment.
In 1959/60, he was involved in an “unfortunate little bloodbath on the border” at the Aburanian border, the country of General Sharp in The Outside-In Man. Steed and a few other agents had been ordered to kill the former British officer Sharp who defected to a terroristic organization in January 1959.

He was in 1961 (has to be after 13th August) in East-Germany, as he tells Purdey, he got shot three times while trying to cross the border in Berlin. The Ministry sent him more than once to the Eastern bloc as the secret service made him go over again as soon as he could walk after his injuries (TNA Obsession). I think that proves that Steed had already become an essential agent for the Ministry. He got shot on at the German border again in the late 1960s (the traitor Crayford returned after ten years to England) when his former friend Mark Crayford tried to kill him near Berlin.

He already worked with Catherine Gale in August 1961 on a job in Berlin (Don’t Look Behind You, Script), as they arrested Martin Gurdman in Germany. He did a lot of work behind the Iron Curtain, as he was in Berlin again in October 1963 where he shot Boris Kartovski (Split).

Before 1963 he worked for a longer time in South-Eastern Europe at the Carinthia Pipeline, an agent network at the Austro-Hungarian, Austro-Jugoslav borders. Steed says he “was on the line for a while” and there is another information about Steed in The Wringer, as he tells that he went through (agents-?) training with Hal Anderson, an old friend of his, one of the few people who call him by his Christian name, John. It’s highly possible that he also went through agent’s training and service with Paul Ryder and George Neville (Get-A-Way!). After the war and before 1963, he was captured at least once by foreign agents, interrogated and tortured as he tells Mrs. Gale in Man with Two Shadows, which obviously implies painful memories. His reaction to the music of Johannes Brahms’ Wiegenlied is very strong in this episode because he is asking Cathy to turn off the music. He was the one and only British agent who got away after four days of imprisonment as Charles tells Mrs. Gale in Man with Two Shadows. John Steed does not like dimly lit rooms (Stay Tuned), maybe that’s caused by his recent imprisonments.

In 1962 he spent some of his holidays in the Mediterranean, maybe Greece (A Chorus of Frogs; Bullseye). He also likes Italy (Stay Tuned), judging from the holiday pictures, even Portugal or Spain (The House that Jack Built) and France (Who’s Who???), where he stayed at Deauville (The Nutshell), Bass-Normandy, the Calvados region - no surprise there.
It looks like his favorite holiday place on the Continent.
He’s on his way to the Middle East in December 1962 (The Big Thinker) as his luggage shows Tel Aviv as a travel aim.

In February 1963, he came back from a mission to Egypt.
“I got back from Cairo last night,” as he tells One Seven (School for Traitors), and this “Cairo job took longer than we thought,” which confirms that he was not alone there. He’s on his way to the Carribean in March 1963 and would like Mrs. Gale to accompany him, but she’s traveling to Tokyo to a judo competition. On 15th May 1963, he was on holiday (The Wringer) with a girlfriend from Alta near Narvik in Norway in a tent. Maybe he spent a holiday in April 1963 with Mrs. Gale at Corfu, as he has some beach photos of her (The Nutshell).

In 1963 Steed owns a “place,” a cottage outside London which is -as he says - “much cozier” than his flat (Lobster Quadrille). He’s on the job in Berlin in October 1963, where he shot Boris Kartovski through the heart (Split!).

For his assignments, during the period he worked with David Keel, Marting King, Venus Smith, Cathy Gale, Emma Peel, and Tara King, go here.

It looks like Steed is also an expert in deciphering codes,  as he’s able to break a secret code in The Springers.

Many episodes prove that he speaks Italian (The Frighteners), French, as he’s reading some Tintin et Milou comics in French, talks French in The Golden Fleece and to “Napoleon” in the QQF episode Honey for the Prince: “Je vous en prie, monsieur; pardon, mon general, mon brave general... je cherche M. Hopkirk”. Steed speaks in Russian “For Peace” in the Nutski’s office in The Correct Way to Kill. He also speaks German (You’ll Catch Your Death) as he went several times over the Wall (TNA Obsession); that is because Steed worked as an agent in East-Germany, but he speaks German less fluent than Dr. Keel (Dead of Winter). He at least understands modern Greek (Game) and has learned Latin (The Grandeur That Was Rome), and I guess classic Ancient Greek at school.

Steed is a connoisseur of (French) wine (Death of a Great Dane, Dial a Deadly Number, The Secrets Broker & The Mauritius Penny).

The biography of John Steed on John Steed’s Flat is based on information found in the episodes of The Avengers.

However there exist fictional biographies for his character.

TV Times September 1963

Tim Heald: John Steed.An authorized biography. Volume 1 - Jealous in Hounour, London 1977
(Judith Galloway's former site Steedophilia has a detailed summary of it's content.)

Roger Davies: 1922 - 19!! A fictionalised biography, in On Target,Volume 2 (1986), Issue 4, pages 3-9

Photo of young Patrick Macnee in his twenties on Patrick Macnee's Facebook site
In 1947 he worked with Peter Borowski, the double agent who has been brainwashed in Man With Two Shadows.

1948 - 1955: unknown

On 8th July 1956 Steed was on a mission to Beirut with an agent called Benson arresting another agent, Malik. Steed is going to Benson’s funeral in Mandrake.

In spring 1958 he lived or worked in Herefordshire (Concerto).
Its' unknown what young Steed did in his late twenties or early thirties, but at least we know what he looked like in 1950.
(photo of Patrick Macnee in the film 'The Girl is Mine' from 1950)
Steed's Biography