[1] Growing up in Ohio, Gillom played for Brown on the football team at Massillon Washington High School, where he excelled as an end, linebacker and punter.

In Gillom's three seasons between 1938 and 1940, Massillon won all of its games and captured two High School Football National Championships.

Gillom followed Brown to Ohio State University in 1941, playing on the school's freshman football team before dropping out because of poor grades.

Gillom's Cleveland teams won three straight AAFC championships before the league dissolved and the Browns were absorbed by the NFL.

Gillom attempted a comeback in 1961 with the New York Titans of the American Football League, but failed to make the team.

Gillom contributed to the evolution of punting by standing further back from the center than was usual at the time to give himself more room to make kicks.

[1][3] Bud Houghton, who coached him when he was in junior high school, said Gillom needed a step and a half more than usual to get his punts off, but he was the best he had ever seen once he made the kick.

[1] Gillom was a favorite of Massillon High football coach Paul Brown, who said in his autobiography that there "has never been a better punter than Horace.

"[1] Gillom played for the Massillon Tigers between 1938 and 1940, a period during which the team won all of its games and two High School Football National Championships.

[4][5][6][7] Gillom, who also played basketball and other sports, earned All-Ohio honors at Massillon and was one of several black players on the team at a time when many northern high schools excluded them.

[2] Gillom played on Ohio State's freshman team that year as an end opposite Dante Lavelli.

[9] Gillom was expected to move up to the varsity team the following year as a blocking back, but was kicked out of school in January for failing to maintain his grades.

[9] Gillom led the nation in punting at Nevada that year, but left the school in December because of poor grades.

[9][14] Gillom signed with the Browns in early 1947, making him the third black player to join the team after Bill Willis and Motley.

[16] "Gillom had such a powerful leg and kicked the ball so far; before that punters used to line up 10, 12 yards behind the center", running back Sherman Howard later said.

The following season, Gillom saw time as an offensive end when regular Dante Lavelli broke his leg and was sidelined for seven games.

The team finished the regular season with a 10–2 record, tied with the New York Giants for first place in the American Conference.

[27] He again led the league the following season, averaging 45.7 yards per punt, and was selected for the Pro Bowl, football's all-star game.

[29] During his years with the Browns, Gillom's long, high punts and his habit of standing further back from center than was usual set a precedent followed by many of his successors at the position.

[32] Gillom's positioning behind the center gave him more space to kick but also put more distance between him and the opposing linemen, reducing the likelihood of a block.