At only eighteen, on July 15, 1938, Lemos defeated prolific Philippine boxer Pablo Dano, holder of the 1935 California World Bantamweight Title, in a ten-round points decision, though he was knocked to the mat twice.

[5] Lemos's father died three months later in November 1938, making Richie the sole supporter of his mother and nine young sisters and brothers.

[4] On April 6, 1939, Lemos lost to Lou Salica, future holder of both the NYSAC and NBA World Bantamweight Titles, at Legion Stadium in Hollywood in a ten-round points decision.

The two young boxers exchanged blows furiously in the last three rounds, with Marcus dropping to his knees briefly in the ninth, perhaps from shear exhaustion.

Fatigued by the tenth, Lemos summoned a last bit of energy to win by a shade in the final round, though he had been previously knocked to the mat on two earlier occasions.

[7] On December 27, 1940, Lemos lost to Petey Scalzo, reigning NBA world featherweight champion, in their first meeting, a non-title bout at Legion Stadium in Hollywood.

A deep cut over one of his Lemos's eyes caused the ring doctor to stop the fight, which led to his losing the bout in a technical knockout in the seventh round.

[8] On April 1, 1941, he defeated Jewish Brooklyn-born boxer Lew Feldman, in a ten-round points decision at Olympic Stadium in Los Angeles.

[9][2] On July 1, 1941, in his first attempt, Lemos took the NBA world featherweight title before a near capacity crowd of 9,500 from Petey Scalzo in a fifth-round knockout of a 12-round bout at the Olympic Auditorium in Los Angeles.

[4] On November 18, 1941, Lemos lost the NBA world featherweight title by a ten-round unanimous decision to Jackie Wilson at Olympic Auditorium in Los Angeles.

[17] On February 3, 1942, Lemos had his third straight loss against Chalky Wright, reigning NYSAC world featherweight champion, at Olympic Auditorium in Los Angeles, when his cut left eye led to a technical knockout in the sixth round.

The following month on April 28, Lemos lost to Lunny in an unpopular ten-round decision before a near capacity crowd in San Francisco's Civic Auditorium.

[2] On July 24, 1942, Lemos lost to 1944 NBA World Lightweight Champion Juan Zurita in a ten-round unanimous decision at Legion Stadium in Hollywood.

[21] On October 5, 1942, Lemos mildly upset Cleo Shans, the betting favorite and number two world lightweight contender, in a ten-round points decision at the Arcadian Arena in Providence, Rhode Island.

[2][22] Lemos also won their final meeting on May 7, 1943, a ten-round points decision by both judges at Legion Stadium in Hollywood, achieving victory using a series of lefts and hooks with both hands.

[24] After his last fight, a ten-round win over Tyree White on August 27, 1943, Lemos felt an intense pain in his eye, later diagnosed as a detached retina.

After his wife gave birth to his last daughter, Joanne, Lemos tried to enlist in the Army but was barred due poor vision, a result of his eye surgery.