The colorful display of regimental precision and pageantry is the first of 74-year-old Charles's reign, and also the first time a ruling monarch will ride at the event since his mother Queen Elizabeth II in 1986.
Charles's actual birthday is on November 14 but British sovereigns celebrate twice -- once in private and again in public, AFP said.
The June parade tradition began in 1748 under King George II, who wanted a celebration in better summer weather, as his own birthday was on October 30.
The televised event gets under way from 10:00 am (0900 GMT), with a procession from Buckingham Palace to Horse Guards Parade in central London.
Some 1,400 soldiers, 400 musicians and 200 horses are taking part, led in the parade by Juno, a 10-year-old shire mare, alongside three other Drum Horses -- Perseus, Atlas and Apollo.
Drum Horses are the most senior animals in the army and hold the rank of major. They are traditionally named after figures from Greek mythology.
The minutely choreographed event has its origins in the display of colors or flags of different regiments to allow their soldiers to identify them in battle.
The 1st Battalion Welsh Guards will troop, or parade, their color up and down the ranks this year.
Highs of 25 degrees Celsius (77 degrees Fahrenheit) are forecast, but troops will likely feel hotter in their ceremonial black bearskin hats and thick red tunics.
Charles's eldest son and heir, Prince William, inspected troops from the Household Division group of senior regiments last weekend, and the high temperatures saw several soldiers faint.
As Prince of Wales, William is honorary colonel of the Welsh Guards.
After inspecting the troops and taking a royal salute, Charles -- who as head of state is commander-in-chief of the armed forces -- will lead soldiers back to the palace.
He will then join other senior members of the royal family to watch a fly-past of some 70 military aircraft over the British capital, after a 41-gun salute from nearby Green Park.
Bad weather cut short a planned fly-past at Charles's coronation on May 6.
Last year's Trooping the Color was the last for the late queen, and formed part of four days of events to mark her record-breaking 70th year on the throne.
It was one of her final public appearances before her death, aged 96, in September.
