After winning his professional debut against the British boxing veteran Bert Gallard in Oslo, Sanstol was invited by Max Schmeling's manager to train in Berlin.

On July 13, 1926 Sanstol defeated Harry Stein, than the Flyweight Champion of Germany, in Berlin at Luna Park in a four-round newspaper decision.

Discovered in Paris by American manager Lew Burston (1896–1969), Sanstol was brought to New York around mid-1927, where he graduated from the club preliminaries to become the most sought after bantam in the eastern United States and Canada.

[7] On October 22, 1930, Sanstol fought a tough ten-round draw against Joey Scalfano at Madison Square Garden that was "nip and tuck for the entire ten rounds", and a great crowd pleaser.

By late 1930, Sanstol had moved his headquarters from the Norwegian-American community of Bay Ridge, in Brooklyn, NY, to Montreal, Canada where he came under the management of Raoul Godbout (1894–1975).

On May 20, 1931, in his most important bout, Sanstol won the World Bantamweight Title in a ten-round unanimous decision against the great New York Jewish contender Archie Bell in Montreal.

Leading in the betting odds by 7 to 5, Sanstol won the fifteen round bout at the Forum in Montreal in a fifteen-round Unanimous Decision.

[2][11] After the win, Sanstol was rated second in the world in the bantamweight division, behind only Panama Al Brown, according to the quarterly standings of the National Boxing Association (NBA).

[12] On July 29, 1931, again at the Forum in Montreal, Sanstol defeated Eugène Huat in a ten-round Unanimous Decision in his second defense of the Canadian version of the World Bantamweight Title.

In the ninth the boxing seemed close, but in the tenth, perhaps with greater stamina, Sanstol took the offensive and battered Huat with telling blows.

[2][13] On August 25, 1931, Sanstol met Panama Al Brown for the widely recognized NBA and NYSAC World Bantamweight Title before a crowd of 23,000 at the Forum in Montreal.

[14] On July 20, 1932 Sanstol fought a ten-round draw with 1929 NBA World Flyweight Champion Emille "Spider" Pladner, a French born boxer, at the Forum in Montreal, Canada.

[2][16] On September 1, 1934, Sanstol defeated World Champion Victor "Young" Perez at Bislet Stadium in Oslo in a ten-round points decision.

[2] With both boxers very near 118, Sanstol had his last NBA World Bantamweight Championship match with Sixto Escobar, on August 7, 1935 at the Forum in Montreal.

In the seventh, Escobar closed Sanstol's left eye, in effect winning the bout, or reducing his need to show the same level of aggression.

[17] Sanstol had one more career bout of consequence, on September 13, 1935, defeating Al Brown in Oslo in a ten-round non-title decision a month after the Escobar fight, not long before his retirement.

During his service, he fought two army benefit fights on May 1, and July 16, 1942 in Miami, Florida, winning both in a four-round and then ten-round decision.

After his first bout with Panama Al Brown, Sanstol learned to pace himself better and to use every punch sparingly, not wasting a single drop of energy.

But fistic age has tempered the pace, has developed a new ring cunning, and a tendency to accomplish by polished skill what he once achieved by youthful energy that disdained to save itself, that was gladly thrown to the winds.

Today Sanstol is inclined to save his legs, to some degree, and to employ instead the ring-craft he has acquired in nearly ten years of campaigning up and down the fistic lanes of two continents.

[22]After his boxing career ended, Sanstol worked various jobs in Norway, New York City, Chicago, Seattle, and Alaska, including restaurant owner, newspaper writer, recreation center director, hotel clerk, and translator.

In 1960, Pete moved back to the West Coast of the United States with his wife, taking a job in Long Beach, California, as a translator for a shipping company.