The company is celebrating the 1968 national launch of the double-decker sandwich whose ingredients of "two all-beef patties, special sauce, lettuce, cheese, pickles, onions and a sesame seed bun" were seared into American memories by a TV jingle.

But the milestone comes as the company reduces its number of US stores as customers visit less often.

The "Golden Arches" still have a massive global reach, and the McDonald's brand of cheeseburgers, chicken nuggets and french fries remains recognisable around the world.

But on its critical home turf, the company is toiling to stay relevant. Kale now appears in salads, fresh has replaced frozen beef patties in Quarter Pounders, and some stores now offer ordering kiosks, food delivery and barista-style cafes.

The Big Mac remains unchanged, showing just how much McDonald's and the rest of fast-food have evolved.

"Clearly, we've gotten a little more sophisticated in our menu development," McDonald's CEO Steve Easterbrook said in a phone interview.

As with many of its popular and long-lasting menu items, the idea for the Big Mac came from a franchisee.

In 1967, Michael James "Jim" Delligatti lobbied the company to let him test the burger at his Pittsburgh restaurants. Later, he acknowledged the Big Mac's similarity to a popular sandwich sold by the Big Boy chain.

"This wasn't like discovering the light bulb. The bulb was already there. All I did was screw it in the socket," Delligatti said, according to Behind the Arches.

The burger was added to the national menu in 1968.

Messing with a signature menu item can be taboo, but keeping the Big Mac unchanged comes with its own risks. Newer chains offer burgers that can make the Big Mac seem outdated.

Periodic promotions should help keep the Big Mac relevant for years to come, says Mike Delligatti, the son of the Big Mac inventor, who died last year.

"What iconic sandwich do you know that can beat the Big Mac as far as longevity?" said Delligatti, himself a McDonald's franchisee.

Australian Associated Press