Rt Rev Richard Chartes is retiring after 21 years in the role.
Archdeacon of London, the Venerable Luke Miller, told Premier a large crowd is expected to enjoy the celebration.
He said: "We hope it's going to be enormous. It's certainly going to be full of life and vigour. We've got two and a half thousand seats in the cathedral; we're hoping for a full cathedral and some hundreds outside as well. Everyone is very welcome."
Miller added that Rt Rev Chartes will leave behind a great legacy.
"He's been an amazing bishop for us over 21 years. Many of us have hardly known any other Bishop of London. His personality and contribution have been amazing," he said, "He's helped us to grow the number of people in the church, we've grown in confidence and deepened in discipleship; he's led us in faithfulness and prayer. He's been an extraordinary bishop."
St Paul's Cathedral will be open to the public from 5.00pm. The service will being at 6.00pm.
The bishop announced his plans to retire in July 2016. He told Premier at the time: "For my part, I have tried to follow the example of St Augustine who said, 'For you I am a bishop but with you I am a Christian', and in this spirit I hope you will forgive my many shortcomings in office."
Listen to Antony Bushfield speaking to the venerable Luke Miller here: