The service will begin at 6pm in the historic building.
According to ITV Royal Editor Chris Ship, 2,000 seats have been made available for members of the public to use.
They are available on a first come first service basis from the City of London tourism office.
Those wishing to attend the service must visit in the office in person at Carter Lane, EC4M 8BX to collect a wristband from 11am.
The cathedral has been closed today in preparation for tonight’s event.
St Paul’s will then reopen on Saturday for private prayer and services.
There will also be a live broadcast of the event on BBC1 this evening.
An order of service will also be available on the St Paul’s website
Lincoln Cathedral will hold a special Evensong at 5.30pm in memory of Her Majesty The Queen, while Chester will hold Evensong at 5.30pm tonight to mark her death.
St Edmundsbury Cathedral in Suffolk will hold a special choral evensong this evening and a Requiem Mass on Sunday.
Dean Jo Kelly-Moore, who chairs the Association of English Cathedrals, said:“The Queen has been a beacon of hope, faith and unity in our country and throughout the world; a constant in the midst of so much change.
“As in the cathedrals of this land we have prayed God’s blessing upon the Queen every day of her reign, we do so again now and we pray for her family at this sad time.
“And we invite you, her people, to join us. All our cathedrals are open so please come and add your voices, and prayers, in remembrance and thanks.”
Cathedrals across the country are taking steps to mourn the Queen.
Muffled bells will ring out from noon, while flags have been lowered to half mast.
Areas will also be set aside for the laying of flowers and books of commemoration.
The English Cathedrals site says the beautiful buildings are “ready to be focal points for a nation in mourning.”
The St Paul’s website also shared a prayer of thanksgiving for the Queen: “Creator of Heaven and Earth,
“through all our days you fashion us and by your grace we grow.
“We thank you for all that was accomplished in your servant Elizabeth.
“As in life she glorified your name, so beyond death may she serve you still;
“through Jesus Christ our Lord. Amen.”