The service held at the iconic St. Paul's Cathedral in London was attended by around 50 royal family members, including the Queen's husband Duke of Edinburgh, who himself is celebrating his 95th birthday today.
British Prime Minister David Cameron gave a reading from the Bible and the Dean of St. Paul's David Ison thanked the Queen for her "dutiful commitment, loving leadership, gentle constancy".
The service was held to pay tribute to her "faithful devotion" to the country.
The Archbishop of Canterbury, Rev. Justin Welby, said in his tribute: "Over the 63 years and the 90 years there has been much to fear: at times of personal challenge or national crisis.
More From This Section
"But just as the Psalmist sees through fear to something more stirring and more extraordinary, so we look back on Your Majesty's 90 years in the life of our nation with deep wonder and profound gratitude.
"Through war and hardship, through turmoil and change, we have been fearfully and wonderfully sustained."
Today marks the start of a whole weekend of celebrations, which will include the annual Trooping the Colour to be held at London's Horse Guards Parade tomorrow, which will see more than 1,400 officers on parade, 200 horses, and more than 400 musicians taking part.
The birthday parade will end with members of the Royal Family making their annual appearance on the balcony of Buckingham Palace, where they will watch a Royal Air Force (RAF) fly-past.
On Sunday, the monarch will host a street party for around 10,000 people at the Patron's Lunch - a celebration of her patronage of more than 600 organisations in the UK and around the Commonwealth.
All proceeds from the sale of tickets for the party will go to charities supported by the royal family.
Britain's Queen always has two birthdays, the official one on the second Saturday of June and her real birthday, which falls on April 21 as part of a tradition dating back nearly 250 years to try to ensure good, sunny weather for the monarch's official celebrations.
Queen Elizabeth II was born on April 21, 1926, to Prince Albert, Duke of York - later King George VI, and Elizabeth Bowes-Lyon and is now the longest-reigning monarch in British history.