Prim's made an immediate impact as a professional in his debut season of 1980, he finished seventh in the early season Tour of Sardinia and was part of the Bianchi team that won the team time trial stage at Paris–Nice, this good form ensured him a ride in the Giro d'Italia where he took a stage into Teramo as well as finishing fourth overall and taking the young riders jersey.
Prim went to the 1981 Giro d'Italia in top form after winning the Tour de Romandie just prior to the Italian race, he ended up taking the runners up spot behind Giovanni Battaglin, many observers felt that Prim could have won the 1981 Giro if his Bianchi squad had backed him solely instead of having three team leaders (Italians Silvano Contini and Giambattista Baronchelli were the others).
Prim finished that year's Giro in fourth place behind three of the cycling world superstars, Bernard Hinault, Francesco Moser and Greg LeMond.
Tommy's reaction to this was to announce his retirement from cycling midway through the 1986 season at the young age of 31, he was feeling tired and felt he could no longer give it 100%.
After his retirement from cycle racing, Tommy opened a bike shop back in his native Sweden, he then moved to other employment working for a mail order firm, a saw mill and then a salmon smokery.