Larsen was a six-time Danish Champion and a Candidate for the World Chess Championship on four occasions, reaching the semifinal three times.

[3] From the early 1970s onward, he divided his years between Las Palmas and Buenos Aires[1] with his Argentinian-born wife Laura Beatriz Benedini.

[7] Furthermore, while at military service, he studied Russian, which was instrumental in assisting him to understand Russian-language chess literature[citation needed] (something Bobby Fischer was also known to have done).

He took first prize at the Gijón International Chess Tournament in 1956, ahead of Klaus Darga and Jan Hein Donner,[14] and in the same year won at Copenhagen with 8/9.

[13] Larsen became an International Grandmaster in 1956 with his gold-medal performance on board one at the Moscow Olympiad, where he drew with World Champion Mikhail Botvinnik.

But he scored his first major individual international success by winning Mar del Plata 1958 with 12/15, ahead of William Lombardy, Erich Eliskases, Oscar Panno, and Hermann Pilnik.

Around this time Larsen diversified his style, switching over to risky and unusual openings in some of his games, to try to throw his opponents off balance; this led to the recovery of his form and further development of his chess.

Larsen's unusual openings were on full display at the 1964 Amsterdam Interzonal, where he shared the 1st–4th places on 17/23 with Boris Spassky, Tal, and Vasily Smyslov, advancing as a Candidate.

Starting in the mid-1960s, Larsen enjoyed a very successful run in major tournaments around the world, and he and Fischer became the two strongest players outside the Soviet Union.

He won at Havana 1967 with 15/19, ahead of a strong group that included Mark Taimanov, Smyslov, Polugaevsky, Gligorić, and Miroslav Filip.

Somewhat unusually for the late 1960s, Larsen—as one of the world's top players—often entered large Open tournaments run on the Swiss system, and had plenty of success.

Larsen won at Monte Carlo 1968 with 9½/13, ahead of Botvinnik, Smyslov, Vlastimil Hort, Robert Byrne, Portisch, and Pal Benko.

This completed a string of five consecutive clear wins of major tournaments, a feat that had not previously been accomplished in modern chess.

In 1988 he lost a game to Deep Thought in the Software Toolworks Championship, becoming the first Grandmaster and, at the time, the player with the highest Elo rating (by then 2560) to be defeated by a computer in tournament play.

According to the English Chess Federation Newsletter, "His health had been poor for some considerable time and he had been virtually inactive for years".

Grandmaster Samuel Reshevsky wrote that Larsen "is a firm believer in the value of surprise", and that led him to resort to "dubious variations in various openings."

He also wrote that while Larsen "has a great deal of confidence in his game and fears no one", his style was "extremely effective against relatively weak opponents but has not been too successful against top-notchers.

He revived the almost dormant Bishop's Opening (1.e4 e5 2.Bc4) with success in 1964 and explored new ways for Black to seek activity in the Philidor Defence (1.e4 e5 2.Nf3 d6).

He was also the first top player to successfully use the Grand Prix Attack against the Sicilian Defence (1.e4 c5 2.f4), spurring a sudden and sustained gain in its popularity.

He played the rare Scandinavian Defence 1.e4 d5 to defeat World Champion Anatoly Karpov in 1979, sparking renewed interest in that variation.

A favourite line in the Caro–Kann Defence (1.e4 c6 2.d4 d5 3.Nc3 dxe4 4.Nxe4 Nf6 5.Nxf6+ gxf6) is co-named for him and David Bronstein; the idea is to accept a weakness to the Black pawn structure in exchange for an unbalancing of the position and retaining the bishop pair.

A chapter of the book gives some of Larsen's thoughts on his own style, and he upholds the views of Polugaevsky and Gligorić that he indulged in flank attacks and favored the advance of rooks' pawns more than other contemporary masters.

He disputed the notion that he would willingly accept dubious positions in order to complicate tactics, a characteristic he attributed more to Tal.

The Grünfeld Defence (1.d4 Nf6 2.c4 g6 3.Nc3 d5) was another opening that became a frequent choice of Larsen with the Black pieces, and similarly he placed considerable reliance on Grünfeld-Indian systems as White.