Argüello debuted on October 26, 1968, trained by former boxer Miguel Angel Rivas. After winning his first 3 fights "The Explosive Thin Man" suffered an unavenged first-round KO loss, followed by another split decision loss. Argüello would then win 29 of his next 30 bouts over the next 5 years, including a win over José Legrá. Eventually, Argüello earned world featherweight championship bout against experienced WBA champion Ernesto Marcel. The fight took place in Panama, Marcel's home country. The young challenger lost a 15-round unanimous decision in the champion's retirement bout. Months after Marcel's retirement, the WBA featherweight title was won by former unified bantamweight champion Rubén Olivares.