One of the strongest non-Soviet players from the early 1960s into the late 1980s, he participated in twelve consecutive Interzonals from 1962 through 1993, qualifying for the World Chess Championship Candidates Cycle a total of eight times (1965, 1968, 1974, 1977, 1980, 1983, 1985, and 1988).

Portisch tied for first through third places in a master event at Budapest 1956, along with József Szily and Béla Sándor, ahead of Pal Benko; the three winners scored 7/11.

Portisch successfully represented Hungary in several team matches in 1956 and 1957, against Poland, Estonia, the Soviet Union, Belarus, and Yugoslavia.

He excelled at his first individual international event, winning at Balatonfüred 1958 with 9/11, ahead of strong Grandmasters László Szabó and Alexander Tolush.

Portisch entered the World Championship cycle for the first time with the Madrid 1960 Zonal, where he tied for second/third place, on 13½/21, along with Arturo Pomar; the winner was Svetozar Gligorić, as all three advanced.

He qualified from the Raach 1969 Zonal, after tying for second/fifth places, on 13½/21, then winning a four-way playoff for two berths at Prague 1970 with 4/6, against Borislav Ivkov, Jan Smejkal, and Ulf Andersson.

He advanced, but narrowly missed Candidates' qualification at the Palma de Mallorca Interzonal 1970, tying with Vasily Smyslov for seventh/eighth places on 13½/23, after another late defeat at the hands of one of the outsiders; the winner was Fischer.

Portisch qualified from the Petropolis Interzonal 1973, scoring 11½/17 for a shared second/fourth place (the winner was Henrique Mecking), then surviving a further three-man playoff for two berths against Lev Polugaevsky and Efim Geller, at Portorož 1973, by winning outright with 5½/8; however, Portisch lost his first-round Candidates' match to Tigran Petrosian in 1974, by 6–7 at Palma de Mallorca.

In the Candidates' matches, he first advanced to the semifinals after beating Bent Larsen by 6½–3½ at Rotterdam 1977, then lost to Boris Spassky by 6½–8½ at Geneva 1977.

Portisch got his revenge against Spassky in 1980, when he tied their quarterfinal match 7–7 in Mexico, and advanced to the semifinals since he had more victories with the Black pieces.

Qualifying from the Toluca Interzonal 1982, where he tied for first/second places on 8½/13 along with Eugenio Torre, he lost his first-round match to Viktor Korchnoi by 3–6 at Bad Kissingen 1983.

He won his first-round match at Saint John, New Brunswick 1988 by 3½–2½ over Rafael Vaganian; he then lost to Jan Timman by 2½–3½ at Antwerp 1989.

In his final appearance in the world championship series, Portisch played well at the 1993 Biel Interzonal, scoring 7½/13 and outperforming his ranking significantly, but did not advance.

"[3] Portisch was very active on the international tournament scene from the late 1950s through the early 1990s, and was one of the top performers for over thirty years, with many titles against elite fields.

His first top-class round-robin event was Moscow 1959, where he was the youngest contestant, and scored 6/11 for a shared fourth/sixth place; the winners were Spassky, Smyslov, and David Bronstein.

Then at Bled 1961, with six of the world's top eleven players in the field, Portisch scored 8/19 for a shared fifteenth/sixteenth place; Tal won.

With six of the world's top 25 in the field at Zagreb 1965, Portisch scored 12/19 for a shared third/fourth place, as Borislav Ivkov and Uhlmann won.

At Santa Monica 1966, with seven of the world's top 16 players in the field, he shared fourth/fifth places on 9½/18, as Spassky won, ahead of Fischer.

Portisch played board three at Belgrade 1970 in the USSR vs Rest of the World match, defeating Viktor Korchnoi by 2½–1½.

He won Wijk aan Zee 1972, which had six of the top 25 players, with 10½/15, ahead of Arturo Pomar, Walter Browne, Vlastimil Hort, and Vasily Smyslov.

Portisch won Las Palmas 1972 with 12/15, ahead of Bent Larsen, Vasily Smyslov, and David Bronstein.

Portisch shared third/fourth places at Palma de Mallorca 1972 on 10/15, with Oscar Panno and Ljubomir Ljubojević winning.

His detailed Olympiad data follows: Portisch has also represented Hungary at eight European Team Championships, winning a total of nine medals.