In his early writings, Mulford portrayed the character as rude, dangerous, and rough-talking. Beginning in 1935, the character—as played by movie actor William Boyd in films adapted from Mulford's books—was transformed into a clean-cut on-screen hero. A total of sixty-six immensely popular films were released, only a few of which relied on Mulford's original story lines. Mulford later revised and republished his earlier works to be more consistent with the character's new, polished on-screen persona.
As portrayed on the screen, the white-haired Bill "Hopalong" Cassidy was usually clad strikingly in black (including his hat, an exception to the longstanding western film stereotype that only villains wore black hats). He was reserved and well spoken, with a fine sense of fair play. He was often called upon to intercede when dishonest characters were taking advantage of honest citizens. "Hoppy" and his white horse, Topper, usually traveled through the west with two companions—one young and trouble prone with a weakness for damsels in distress, the other comically awkward and outspoken.
The juvenile lead was successively played by James Ellison, Russell Hayden, George Reeves, and Rand Brooks. George Hayes (later to become known as "Gabby" Hayes) originally played Cassidy's grizzled sidekick, Windy Halliday. After Hayes left the series because of a salary dispute with producer Harry Sherman, he was replaced by the comedian Britt Wood as Speedy McGinnis and finally by the veteran movie comedian Andy Clyde as California Carlson. Clyde, the most durable of the sidekicks, remained with the series until it ended. A few actors of future prominence appeared in Cassidy films, most notably Robert Mitchum, who appeared in seven of the films at the beginning of his career.
The sixty-six Hopalong Cassidy pictures were filmed by independent producers who released the films through the studios. Most of the "Hoppies," as the films were known, were distributed by Paramount Pictures to highly favorable returns. They were noted for their fast action and excellent outdoor photography (usually by Russell Harlan). Harry Sherman was anxious to make more ambitious movies and tried to cancel the Cassidy series, but popular demand forced Sherman to go back into production, this time for United Artists release. Sherman gave up the series once and for all in 1944, but William Boyd wanted to keep it going. To do this, he gambled his entire future on Hopalong Cassidy, mortgaging virtually everything he owned to buy both the character rights from Mulford and the backlog of movies from Sherman.
In the first film, Hopalong Cassidy (then spelled "Hop-along") got his name after being shot in the leg. Hopalong's "drink of choice" was the nonalcoholic sarsaparilla.
Television and radio
Boyd resumed production  in 1946, on lower budgets, and continued through 1948, when "B" westerns in general were being phased out. Boyd thought that Hopalong Cassidy might have a future in television, spent $350,000 to obtain the rights to his old films, and approached the fledgling NBC television network. The initial broadcasts were so successful that NBC could not wait for a television series to be produced and simply reedited the old feature films down to broadcast length. On June 24, 1949, Hopalong Cassidy became the first network Western television series.
The enormous success of the television series made Boyd a star. The Mutual Broadcasting System began broadcasting a radio version of Hopalong Cassidy, with Andy Clyde (later George MacMichael on Walter Brennan's ABC sitcom The Real McCoys) as the sidekick, in January 1950; at the end of September, the show moved to CBS Radio, where it ran until 1952.
The series and character were so popular that Hopalong Cassidy was featured on the cover of national magazines, such as Look, Life, and Time. Boyd earned millions as Hopalong ($800,000 in 1950 alone), mostly from merchandise licensing and endorsement deals. In 1950, Hopalong Cassidy was featured on the first lunchbox to bear an image, causing sales for Aladdin Industries to jump from 50,000 units to 600,000 units in just one year. In stores, more than 100 companies in 1950 manufactured $70 million of Hopalong Cassidy products, including children's dinnerware, pillows, roller skates, soap, wristwatches, and jackknives.
There was a new demand for Hopalong Cassidy features in movie theaters, and Boyd licensed reissue distributor Film Classics to make new film prints and advertising accessories. Another 1950 enterprise saw the home-movie company Castle Films manufacturing condensed versions of the Paramount films for 16-mm and 8-mm projectors; they were sold through 1966. Also, in January 1950 Dan Spiegel began to draw a syndicated comic strip with scripts by Royal King Cole; the strip lasted until 1955.
Boyd began work on a separate series of half-hour westerns made especially for television; Edgar Buchanan was his new sidekick, Red Connors (a character from the original stories and a few of the early films). The theme music for the television show was written by veteran songwriters Nacio Herb Brown (music) and L. Wolfe Gilbert (lyrics). The show ranked number 7 in the 1949 Nielsen ratings. The success of the show and tie-ins inspired several juvenile television westerns, such as The Range Rider, Tales of the Texas Rangers, Annie Oakley, The Gene Autry Show, and The Roy Rogers Show.
Boyd's company devoted to Hopalong Cassidy, U.S. Television Office, is still active and, in addition to holding the copyrights to the Cassidy series, has released many of the features to DVD, many of them in sparkling prints prepared by Film Classics.
On May 26, 1951, an amusement park named Hoppyland opened in the Venice section of Los Angeles. This was an expansion and retheming of Venice Lake Park (opened the previous year) as Boyd became an investor. Standing on some 80 acres (320,000 m2) it included a roller coaster, miniature railroads, pony rides, boat ride, Ferris wheel, carousel, and other thrill rides along with picnic grounds and recreational facilities. Despite Boyd's regular appearances as Hoppy at the park, it was not a success and shut down in 1954.
In other media
Louis L'Amour wrote four Hopalong Cassidy novels, which are still in print. In 2005, author Susie Coffman published Follow Your Stars, containing new stories starring the character. In three of these stories, Coffman has written the wife of actor William Boyd into the stories.
Comic books/comics strips
Fawcett Comics published a Hopalong Cassidy comic book one-shot in 1943, followed by an ongoing series from 1946–1953, when the company ceased publishing. DC Comics took over the title in 1954 with issue #86, publishing it until issue #135, in 1959.
Mirror Enterprises Syndicate distributed a Hopalong Cassidy comic strip starting in 1949; it was bought out by King Features in 1951, running until 1955. The strip was drawn by Dan Spiegle.
The classic song "It's Beginning To Look A Lot Like Christmas" includes a reference to Hopalong boots as a holiday gift desired by children.
There have been a number of museum displays of Hopalong Cassidy. The major display is at the Autry National Center at Griffith Park in Los Angeles, California. Fifteen miles east of Wichita, Kansas, at the Prairie Rose Chuckwagon Supper was the Hopalong Cassidy Museum. This museum was dedicated to the heroic image of Hopalong Cassidy. The museum and its contents were auctioned on August 24, 2007, owing to the failure of its parent company, Wild West World.
On June 16, 2009, Echo Bridge Home Entertainment released the Hopalong Cassidy Ultimate Collector's Edition, which included all 66 theatrical films on 14 DVDs, packed into a facsimile Hopalong Cassidy tin lunchbox.
- "William Boyd & Hopalong Cassidy", B-Westerns
- "Kiddies in the Old Corral" Time, 27 November 1950.
- Broadcast Firsts – Western Series
- Radio Broadcast Log Of: Hopalong Cassidy part of Audio Classics Archive. Retrieved 2006-12-09.
- Westerns – Museum of Broadcast Communications
- Hopalong Cassidy Returns
- Dan Spiegle
- "Bill Boyd Will Appear at Hoppyland Opening" Billboard March 31, 1951 p.52
- Hopalong Cassidy (1943 series) at the Grand Comics Database
- Hopalong Cassidy (1946 series) at the Grand Comics Database
- Irvine, Alex; Dolan, Hannah, ed. (2010). "1950s". DC Comics Year By Year A Visual Chronicle. Dorling Kindersley. p. 72. ISBN 978-0-7566-6742-9. "Following the decision to close the comics division of Fawcett Publications in 1953, Hopalong Cassidy came to DC with issue #86...by the writers Gardner Fox and Don Cameron and artist Gene Colan."
- Hopalong Cassidy (1954 series) at the Grand Comics Database
- Alan Livingston and The Capitol Record Readers
- Caro, Joseph, Collector's Guide to Hopalong Cassidy Memorabilia (1991, out of print)
- Caro, Joseph, Hopalong Cassidy Collectibles. CCN Publishing (1998) – 1,300 color photos and item conditions
- Drew, Bernard A., The Hopalong Cassidy Radio Program. Albany: BearManor Media (2005) ISBN 1-59393-006-2
- Hall, Roger, Following the Stars: Music and Memories of Hopalong Cassidy. Stoughton: PineTree Press (2005)
- Perham, Joseph A., Reflections on Hopalong Cassidy: A Study of Clarence E. Mulford, unplublished M.A. thesis, University of Maine (1966)
- Spiegle, Dan and Royal King Cole, Paragon Publications Presents Clarence E. Mulford's Hopalong Cassidy and the Five Men of Evil. A.C. Comics (Jan. 1993) ISBN 978-1-56225-002-7 (comic strip reprint collection)