A native of Long Beach, Deniston attended St. Mary's College of California for one year before starting a professional baseball career in 1939.

He played for several minor league teams before serving in World War II at the San Diego Marine Corps Base.

After the war was over Deniston returned to professional baseball as a player-manager, spending 1947 to 1955 both playing for and managing several minor league teams while also serving as a college coach at Pepperdine and Drake University.

[2] Deniston was described by The Long Beach Sun as "an outstanding end and one of the best pass grabbers in the Southland prep ranks.

"[3] Deniston attended St. Mary's College of California for one year, in 1938, before accepting an offer to play professional baseball by the New York Yankees.

[2] Deniston briefly was a boxer in 1940, and won the California State Golden Gloves championship in the lightweight division that year.

[5] In 1944, he was drafted to serve in World War II, and trained at the San Diego Marine Corps Base.

[16] That year he played football for the El Toro Flying Marines military service team that compiled a 8–1 record and was ranked number sixteen in the country.

[5] In 1945, after being released by the Marines, Deniston returned to the Hollywood Bears in the PCPFL, playing the halfback and quarterback positions.

[17] While in the Marines, bone chips and calcium deposits had weakened Deniston's arm, and he "had given up hope of advancing as a baseball player.

"[2] Despite this, he continued playing, with the Pepperdine Waves baseball team in 1945, and scored a home run in their 20–14 win over UCLA.

[21] He appeared in a total of 29 games, and was at-bat 107 times, making 35 hits, 11 doubles, one triple, and four home runs.

[8][2] Deniston appeared in 65 games, mainly as a catcher, and was at-bat 203 times, making 54 hits, seven doubles and seven home runs.

[25] The Bruin manager Al Todd said that "he hated to see Deniston leave," but that "with recent player acquisitions, Des Moines was above the maximum limit for salaries paid by a Class-A club.

"[25] After being released, Deniston was signed by the Colorado Springs Sky Sox of the same league, who needed a catcher after injuries to all their players at the position.

[27] In 1952, Deniston played catcher for the Kellogg Cardinals, and by July held the league lead in batting average with .485.

[36] After finishing the season with them, Deniston retired and accepted a coaching position at La Jolla High School.

In 1946, after returning from World War II, Deniston assisted their football team while also playing the sport professionally.

[5] Also in 1947, Deniston was assigned by the St. Louis Browns to play for and manage the Mayfield Clothiers minor league team.

[22][10] But, as "herding a bunch of youngsters (the average Olean player age was 19 years old) poses pitfalls not encountered in higher leagues," the Oilers only won 39 of 125 games under Deniston's leadership, just a .312 winning percentage.

[23] After Audubon compiled an 18–21 Iowa State League record to start the season, he resigned for "the best interest of the team.

[30] For the start of the 1954 season, Deniston played for New Ulm, before suffering an injury that made him return to Storm Lake as manager.

[35][5] After finishing the season with them, Deniston retired and accepted a position as head football coach of La Jolla High School.

[1] His 1965 team posted a 10–1 record and won the San Diego Section championship game against Point Loma.

[1] In 1967, he led them to another 10–1 record with quarterback Jerry Powell, who later played professionally, outscoring opponents 313–75 and winning the championship game.

Deniston later returned to Lincoln High School as head baseball coach, spending 1985 to 1986 in that position before retiring.