During his time at Navy, he was a two-time consensus All-American and set an NCAA record with 7,172 career all-purpose yards.

He was a member of the Navy Midshipmen football team that played in the Army-Navy game at the Rose Bowl stadium in Pasadena, California in 1983.

In May, McCallum was selected in the 1986 USFL draft by the Baltimore Stars in the third round after a trade in the previous year with the San Antonio Gunslingers.

[6] In 1986 the Navy assigned him to USS Peleliu (LHA-5), home ported in Long Beach, California, near Los Angeles, as a supply officer.

[7] He played for the Los Angeles Raiders while on active duty that year; the military does not prohibit outside employment that does not interfere with service.

[8] He rushed for his season-high NFL totals with 536 yards while splitting playing time with incumbent Marcus Allen.

The Navy assigned McCallum in 1987 to USS California (CGN-36), home ported in Alameda, California and at that time sailing in the Indian Ocean, preventing him from returning to the NFL;[8] McCallum believed that the Navy did so because of the attention he received from playing for the Raiders.

On September 5, 1994, during a Monday Night Football contest and the Raiders' opening game of the season at the San Francisco 49ers, McCallum's career ended prematurely when 49ers linebacker Ken Norton Jr. twisted him to the ground.

He initially thought he'd only face a lengthy rehab, but McCallum's surgeon told him that there was no chance of him ever being medically cleared to play again.