Seaman was born into a wealthy English family in Aldingbourne House near Chichester, Sussex. He grew up partly at Kentwell Hall in Suffolk and attended Rugby School before going to Trinity College, Cambridge. His parents encouraged careers in law or politics, but from 1934 onward he resolved to become a racing driver, taking his MG to Europe to gain experience on continental circuits.
Seaman won the Voiturette race at the Swiss Grand Prix at Bremgarten on his first attempt and went on to win it three times consecutively. Racing with English Racing Automobiles (ERA), he won at Brooklands and Donington Park and took pole position at the 1935 Dieppe Grand Prix. He also won the junior class at the 1935 Coppa Acerbo and the Czech Masaryk Grand Prix that September.
His 1936 season was particularly strong. He dominated with a 1926 Delage that had been extensively developed and was described as nearly unbeatable at the time. He won the British Empire Trophy at Donington Park in an ERA, and won the 1936 Donington Grand Prix in the Delage, sharing the result with Hans Ruesch. He also impressed at Freiburg hillclimbing, finishing only a second behind Hans Stuck overall at the ADAC Schauinsland event.
These results attracted the attention of Mercedes team chief Alfred Neubauer, who invited Seaman for a trial at the Nürburgring. His mother strongly opposed him signing for a team that was effectively part of the Nazi government's prestige racing program, but in 1937 Seaman joined Mercedes against her wishes. The Mercedes and Auto Union cars — both recipients of German government subsidy — were far faster and better financed than anything he had previously driven.
Seaman's first season at Mercedes, 1937, began poorly. He was involved in the fatal accident of Ernst von Delius at the German Grand Prix and injuries kept him out of subsequent races. He finished fourth at the Italian Grand Prix and repeated the performance at the non-championship Czechoslovakian Grand Prix at Brno. Neubauer demoted him to reserve status for the Swiss Grand Prix. Outside Europe, he finished second to Bernd Rosemeyer at the Vanderbilt Cup in Long Island.
The 1938 season brought his greatest triumph. At the German Grand Prix — the most important race of the year for the German teams — Seaman won outright, becoming the first British driver to win a European Championship race in fifteen years. He was subsequently cited as a favorite driver of Adolf Hitler. On the return from his peak, Seaman took pole and finished second at the Swiss Grand Prix at Bremgarten, a drive his biographer George Monkhouse called the finest of his career given the difficult wet conditions. He finished third at the British Grand Prix at Donington Park.
In 1939, Seaman had a slow start to the season. Leading the Belgian Grand Prix at Spa-Francorchamps in wet conditions during lap 22, he took a line through a corner that was suited only to dry weather, lost control, and crashed into a tree. The car caught fire with Seaman unconscious inside. He died a few hours later from burns. On his deathbed he reportedly told Mercedes' chief engineer: "I was going too fast for the conditions — it was entirely my own fault. I am sorry."
Following his death, Mercedes-Benz dealerships worldwide were ordered to display his photograph in their windows. He was buried at Putney Vale Cemetery in London. A memorial stone was later installed at Spa-Francorchamps; it subsequently went missing and was never found.
Seaman married Erica Popp, daughter of the director of BMW, in December 1938, against his mother's opposition. He was twenty-five and she was eighteen. His new father-in-law gave the couple a house in Bavaria as a wedding gift.
He maintained a difficult relationship with his mother throughout his life, partly because of her opposition to his Mercedes contract. Within the team, Seaman got along well with teammates Manfred von Brauchitsch and Rudolf Caracciola, who shared his aristocratic social background. In his autobiography, Hermann Lang described Seaman as "kind-hearted, cool and fair as a sportsman."
Seaman is regarded alongside Henry Segrave as one of Britain's finest prewar Grand Prix drivers. His legacy carries a degree of controversy, however. He was privately complimentary about Hitler and gave a salute — described by observers as "distinctly unemphatic" — following his victory at the 1938 German Grand Prix. The largest wreath at his funeral was sent by Hitler, against the wishes of his family. His biographer Richard Williams has argued that Seaman simply wanted to drive the fastest car available, irrespective of politics, but his association with the Nazi racing program has remained a point of historical discussion.