During the swimming scene at the pool Julie Newmar's character was supposed to be topless with a loincloth. In an interview Ms. Newmar stated that at the last minute she decided to do the scene nude and no-one, especially the male actors and crew, argued with her about the decision.
The film was originally planned to be shown in single lens Cinerama with reserved seat roadshow engagements, but Columbia execs changed their minds and pulled the plug on that idea. The film was drastically cut down from nearly three hours plus an intermission to just over two hours. Although most of the film was photographed on 65mm stock, to save a buck, a handful of scenes were filmed in 35mm anamorphic and then optically blown up with disastrous results. The blown-up scenes are exceedingly grainy and have bad color.
One of George Lucas's projects while a student in film school was documenting the making of this film. While he was on set, he actually made suggestions (which were used) on how to rehearse and shoot some scenes.
When the raft/ferry carrying the treasure seekers across the river is cut free it's swept down stream and everyone jumps off and swims ashore. There's an aerial shot of the raft supposedly breaking up in the rapids then a shot looking up at a waterfall as the wreckage comes over the top. The waterfall is actually High Force in Teesdale, Co Durham, England.
Although not well received by critics and audiences, Mackenna's Gold was released internationally to some success.The film was popular in the Soviet Union. It was first shown at the VIII Moscow International Film Festival in 1973, followed by a cinematic premiere in 1974. The film was viewed by 63 million people and now stands fourth in the all-time rating of a foreign film distribution in the Soviet Union. The title song "Old Turkey Buzzard" was dubbed with Russian lyrics by Leonid Derbenyov, a Russian poet and lyricist widely regarded as one of the stalwarts of 20th-century Soviet and Russian pop music. It was performed by then-popular Soviet singer Valery Obodzinsky.
Although the novel title and hero shared the same spelling of the name "Mackenna," and the film's title according to the studio is "Mackenna's Gold," Gregory Peck's character is listed in publicity materials as "MacKenna."