[3] In 1925, when ten-round fights were legalized in California, Callahan ran through a number of opponents, including Russel LeRoy and Pal Moran, knocking both out in three rounds.

Callahan defended his title again on May 31, 1927, against Spug Myers at Wrigley Field in Chicago and then on May 28, 1929, against Fred Mahan in Los Angeles.

The NBA again recognized the ten-round points decision win over Myers as for the World Super Lightweight title.

[7] Of his March 28, 1928, ten-round win over Dick Hoppe at the Olympic Auditorium, the Montreal Gazette wrote, that he had won "in the opinion of ring siders seven of the ten rounds."

On February 18, 1930, Callahan put his title on the line against "Kid" Berg at Royal Albert Hall in London, England.

The New York State Athletic Commission recognized the Berg–Callahan fight as being for the Light Welterweight title, however, and Berg won NBA recognition in 1931.

[9] After gaining a decade of experience as a referee, Callahan simultaneously worked as a boxing judge for over 100 fights mostly in the Los Angeles area, including four state championships, between 1941 and 1960.

[10] On October 9, 1937, Callahan was scheduled to box in a benefit exhibition at Legion Stadium in Los Angeles for Wad Wadheim, a fight promoter who had suffered a stroke.

Among the 50 or so boxers to participate were triple World Champion Henry Armstrong, Callahan's opponents Jackie Fields, Ace Hudkins, and Jack Silver, as well as Young Jack Thompson, Abe "The Newsboy" Hollandersky, Fidel La Barba, ex-Middleweight champion Al McCoy, Jim Jeffries, and Maxie Rosenbloom.

[15][4] As a stuntman on Paramount's 1953 War of the Worlds, Callahan suffered burns while performing one of the first full-body fire stunts on film.

[16][17] Among the actors he trained were Montgomery Clift (From Here to Eternity), James Dean (Rebel Without a Cause), Kirk Douglas (Champion), Errol Flynn (Gentleman Jim), James Earl Jones (The Great White Hope), Burt Lancaster (Jim Thorpe – All-American), Anthony Quinn (Requiem for a Heavyweight), and Elvis Presley (Kid Galahad).

[22][23] In 1934, Callahan married Leonora Summers (née Lillian Hill, 1897–1976[24]), a former silent film actress, in Los Angeles.