World number three Martin, an Australian by birth, will ride the first of 74 dressage tests scheduled over two days.
Another storied rider, double gold medallist Mark Todd of New Zealand, who came back to eventing in 2008 after eight years of retirement, will wrap up the running order on Sunday afternoon.
Despite the illustrious field on Saturday, session two on Sunday is likely to attract more feverish media attention.
That's when British royal Zara Phillips, granddaughter of Queen Elizabeth II and daughter of two Olympians -- Princess Anne and Mark Phillips -- will ride out on her horse High Kingdom.
The Duke and Duchess of Cambridge and Prince Harry plan to be on hand to watch their royal cousin. Phillips, 31, is competing at her first Olympics after missing out on the Games in Athens and Beijing due to fitness concerns over her horse Toytown.
The home team's brightest hope, top-ranked William Fox-Pitt will be third-last to go.
Will Connell, British Equestrian Team leader, said he expected a hard-fought contest over the four days but that the experience of riders such as Fox-Pitt would help.
"We have five extremely talented riders in eventing, all of whom have won medals at European and/or world and Olympic level," he told Reuters in an interview.
"And I think that this (cross-country) terrain here will require riders with experience and I think those that are younger and more inexperienced are going to find this pretty difficult."
Once the horse-rider pairs from 22 nations have shown they can execute dressage's exacting movements, they will go full throttle in Monday's cross-country -- over a hilly 5.76 kilometre track of solid fences winding through Greenwich Park.
On Tuesday, it will be time for care again, when riders take their mounts over more delicately balanced fences in the jumping competition.
Sue Benson, who designed the cross-country course, agreed the terrain and the time would be challenging for riders.
When asked how many she thought would finish jumping the 28 obstacles without incurring penalities for taking longer than the optimum time of 10 minutes and three seconds, Benson said: "I'm going to stick my neck out and say two."
Mistakes in any of the three legs notch up penalties. The rider with the fewest penalties wins.
With an eye to encouraging the spread of eventing around the world, she sought to build a series of fences challenging enough for a "worthy winner" but not so challenging that those from nations less experienced in eventing would come to grief.
"I never wanted any seriously unpleasant pictures beamed across the world," she said.
Still, some of the riders walking the course on Friday looked daunted by a few of the obstacles, which feature replicas of everything from a river barge to the moon.
French team member Lionel Guyon said several looked tough.
"All the ones with flags (that are) very narrow," he said. "It's a new generation of course -- more and more narrow, but not so dangerous."
Like the riders who will face them on Monday, each of Benson's cross-country obstacles has a story -- from the ancient Roman market to the fence featuring 22 working clocks, showing the correct time of each competitor nation.
Not that any rider will have time to check.
- Zara Phillips
- Mark Phillips